RFR: 8264886: WebKit native to Java invocations on wrong thread

[Johan Vos]

Fix for JDK-8264886

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Commit messages:
 - Use env->AttachCurrentThread before making invocations from native to Java

Changes: https://git.openjdk.java.net/jfx/pull/459/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=459&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264886
  Stats: 21 lines in 1 file changed: 11 ins; 1 del; 9 mod
  Patch: https://git.openjdk.java.net/jfx/pull/459.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/459/head:pull/459

PR: https://git.openjdk.java.net/jfx/pull/459