RFR: 8313177: Web Workers timeout with Webkit 616.1

[Jay Bhaskar]

Issue: Some web worker tests fail to finish.
Root Cause:
sharedTimerFiredInternal function from ThreadTimers class does not require an isMainThread check, The check was introduced during the initial webkit stabilization.

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Commit messages:
 - 8313177: Web Workers timeout with Webkit 616.1

Changes: https://git.openjdk.org/jfx/pull/1191/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1191&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313177
  Stats: 7 lines in 1 file changed: 0 ins; 7 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1191.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1191/head:pull/1191

PR: https://git.openjdk.org/jfx/pull/1191