RFR: 8347937: Canvas pattern test fails on WebKit 620.1

[Sergey Nazarkin]

Hi!

This is my desperate fix for what I believe to be a critical bug. Since my knowledge of the codebase is minimal, and the class structure is a bit controversial, my fix is mostly based on historical excavation.

#  What is the problem?
VM started to crash after webkit 620.1 update when running simple [CanvasTest](https://github.com/openjdk/jfx/blob/1c3cfcb8bb4e8874406d3a5b507f38859f4c1d9b/modules/javafx.web/src/test/java/test/javafx/scene/web/CanvasTest.java#L125).  Running with `-Xcheck:jni ` reveals invalid object is passed to JNI [call](https://github.com/openjdk/jfx/blob/0555fb25a16b6b6705a42c6d8592cf1c6ddccc67/modules/javafx.web/src/main/native/Source/WebCore/platform/graphics/java/NativeImageJava.cpp#L56): expected is`WCImageFrame` but provided is `RTImage` (an descendant of `WCImage`). The latter has no `getSize` method.
Stack trace

    frame #4: 0x0000000101a36094 libjvm.dylib`checked_jni_CallObjectMethodV(env=0x0000000124a89cb0, obj=0x000000012310f9e8, methodID=0x00006000019abde8, args="\xa0\U00000012\U0000001ev\U00000001") at jniCheck.cpp:1012:1 [opt]
    frame #5: 0x000000033e88ea60 libjfxwebkit.dylib`JNIEnv_::CallObjectMethod(this=0x0000000124a89cb0, obj=0x000000012310f9e8, methodID=0x00006000019abde8) at jni.h:906:18
    frame #6: 0x00000003426fcda0 libjfxwebkit.dylib`WebCore::PlatformImageNativeImageBackend::size(this=0x0000600000e12790) const at NativeImageJava.cpp:55:48
    frame #7: 0x000000034258d76c libjfxwebkit.dylib`WebCore::NativeImage::size(this=0x00000001761c2c00) const at NativeImage.cpp:89:23
    frame #8: 0x00000003425723c0 libjfxwebkit.dylib`WebCore::ImageFrame::ImageFrame(this=0x00000001761e1270, nativeImage=0x000000016fdf7090) at ImageFrame.cpp:40:29
    frame #9: 0x0000000342572420 libjfxwebkit.dylib`WebCore::ImageFrame::ImageFrame(this=0x00000001761e1270, nativeImage=0x000000016fdf7090) at ImageFrame.cpp:39:1
    frame #10: 0x00000003425a4d7c libjfxwebkit.dylib`WebCore::NativeImageSource::NativeImageSource(this=0x00000001761e1260, nativeImage=0x000000016fdf7090) at NativeImageSource.cpp:37:7
    frame #11: 0x00000003425a4d24 libjfxwebkit.dylib`WebCore::NativeImageSource::NativeImageSource(this=0x00000001761e1260, nativeImage=0x000000016fdf7090) at NativeImageSource.cpp:38:1
    frame #12: 0x00000003425a4cac libjfxwebkit.dylib`WebCore::NativeImageSource::create(nativeImage=0x000000016fdf7090) at NativeImageSource.cpp:33:26
    frame #13: 0x00000003423a6f18 libjfxwebkit.dylib`WebCore::BitmapImage::BitmapImage(this=0x00000001761e1200, image=0x000000016fdf7090) at BitmapImage.cpp:67:16
    frame #14: 0x00000003423a6b60 libjfxwebkit.dylib`WebCore::BitmapImage::BitmapImage(this=0x00000001761e1200, image=0x000000016fdf7090) at BitmapImage.cpp:68:1
    frame #15: 0x00000003423a6bd0 libjfxwebkit.dylib`WebCore::BitmapImage::create(nativeImage=0x000000016fdf70e0) at BitmapImage.cpp:52:26
    frame #16: 0x00000003416abcf0 libjfxwebkit.dylib`WebCore::HTMLCanvasElement::copiedImage(this=0x0000000321008640) const at HTMLCanvasElement.cpp:903:29
    frame #17: 0x0000000341899e70 libjfxwebkit.dylib`WebCore::CanvasRenderingContext2DBase::createPattern(this=0x0000000321008b00, canvas=0x0000000321008640, repeatX=true, repeatY=true) at CanvasRenderingContext2DBase.cpp:2232:32 


# Historical excavation
There was a similar problem in the [past](https://hg.openjdk.org/openjfx/10-dev/rt/rev/542116615e59).  The predecessor of `PlatformImageNativeImageBackend::size` knew that there is an `Image` (like gif with multiple frames, or still image) and ImageFrame.  But then it assumes "NativeImage" is always backed by `WCImageFrame`

# The fix
The fix restores original idea and checks the backend class.  The method returns zero size if the `m_platformImage` is not `WCImageFrame`


# The alternative
I believe there is a more straightforward and correct way to fix this. Don't hesitate to reject the PR if you know a better solution.

-------------

Commit messages:
 - 8347937: Canvas pattern test fails on WebKit 620.1

Changes: https://git.openjdk.org/jfx/pull/1734/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1734&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347937
  Stats: 19 lines in 3 files changed: 12 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1734.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1734/head:pull/1734

PR: https://git.openjdk.org/jfx/pull/1734