RFR: 8263788: JavaFX application freezes completely after some time when using the WebView

[Arun Joseph]

Issue: Java application (with WebView) will completely freeze after using it for a while.

Fix: Use native isMainThread functions instead of JNI call.

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Commit messages:
 - Remove platform check in ThreadingWin
 - Remove fwkIsMainThread
 - 8263788: JavaFX application freezes completely after some time when using the WebView

Changes: https://git.openjdk.java.net/jfx/pull/461/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=461&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263788
  Stats: 38 lines in 3 files changed: 15 ins; 18 del; 5 mod
  Patch: https://git.openjdk.java.net/jfx/pull/461.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/461/head:pull/461

PR: https://git.openjdk.java.net/jfx/pull/461