RFR: 8088420: JavaFX WebView memory leak via EventListener [v2]

[Jay Bhaskar]

> This PR is new implementation of JavaEvent listener memory management.
> Issue  [JDK-8088420](https://bugs.openjdk.java.net/browse/JDK-8088420?filter=-1)
> 
> 1. Calling remove event listener does not free jni global references.
> 2. When WebView goes out of scope (disposed from app) , its Event Listeners are not being garbage collected.
> 
> Solution:
> 1.  Detached the jni global reference from JavaEventListener.
> 2. Create scoped ref counted wrapper class JavaObjectWrapperHandler for jni global reference.
> 3. Create unique  JavaObjectWrapperHandler object for each JavaEventListener.
> 4. EventListenerManager is a singleton class , which stores the JavaObjectWrapperHandler mapped with JavaEventListener.
> 5. EventListenerManager also stores the JavaEventListener mapped with DOMWindow.
> 6. When Event listener explicitly removed , JavaEventListener is being forwarded to EventListenerManager to clear the listener.
> 7. When WebView goes out of scope, EventListenerManager will de-registered all the event listeners based on the ref counts attached with WebView DOMWindow.

Jay Bhaskar has updated the pull request incrementally with one additional commit since the last revision:

  Platform.exit() , removing code block, as it is causing other test fail

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/799/files
  - new: https://git.openjdk.java.net/jfx/pull/799/files/a62558ea..2d5604bf

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=799&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=799&range=00-01

  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/799.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/799/head:pull/799

PR: https://git.openjdk.java.net/jfx/pull/799