RFR: 8234593: Mark LeakTest.testGarbageCollectability as unstable

[Arun Joseph]

The test.javafx.scene.web.LeakTest.testGarbageCollectability unit test is failing intermittently.

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Commits:
 - 1f7c5c8e: 8234593: Mark LeakTest.testGarbageCollectability as unstable

Changes: https://git.openjdk.java.net/jfx/pull/49/files
 Webrev: https://webrevs.openjdk.java.net/jfx/49/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8234593
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/49.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/49/head:pull/49

PR: https://git.openjdk.java.net/jfx/pull/49