RFR: 8301712: [linux] Crash on exit from WebKit 615.1

[Jay Bhaskar]

Issue: [linux] Crash on exit from WebKit 615.1 Crash dump observed on Window exit
Root cause: Accessing local storage data of a thread having client data like font cache during VM shutdown
Solution: Add a shutdown hook to ThreadTimers class, so that no timer class doesn't fire any function during the shutdown.

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Commit messages:
 - [linux] Crash on exit from WebKit 615.1

Changes: https://git.openjdk.org/jfx/pull/1027/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1027&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301712
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1027.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/1027/head:pull/1027

PR: https://git.openjdk.org/jfx/pull/1027