RFR: 8244579: Windows "User Objects" leakage with WebView

[Arun Joseph]

Cause: The Window Class `RunLoopMessageWindow` is never registered (this happens because
registerRunLoopMessageWindowClass() is moved to MainThreadWin.cpp while openjfx uses MainThreadJava.cpp) and this
causes every setTimer() to create a new user object instead of reusing the same object over again.

Fix: Call registerRunLoopMessageWindowClass() from MainThreadJava.cpp

Test: To verify open any webpage using WebView on WIndows, with and without the fix and compare the number of user
objects created using Windows Task Manager.

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Commit messages:
 - 8244579: Windows "User Objects" leakage with WebView

Changes: https://git.openjdk.java.net/jfx/pull/229/files
 Webrev: https://webrevs.openjdk.java.net/jfx/229/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244579
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/229.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/229/head:pull/229

PR: https://git.openjdk.java.net/jfx/pull/229