[jfx11u] RFR: 8264990: WebEngine crashes with segfault when not loaded through system classloader

[Kevin Rushforth]

Clean backport to jfx11u. Tested on all three platforms, along with my other in-progress backports, all of which are aggregated in my [`kevinrushforth:test-kcr-11.0.12`](https://github.com/kevinrushforth/jfx11u/commits/test-kcr-11.0.12) branch.

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Commit messages:
 - 8264990: WebEngine crashes with segfault when not loaded through system classloader

Changes: https://git.openjdk.java.net/jfx11u/pull/11/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=11&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264990
  Stats: 342 lines in 7 files changed: 315 ins; 7 del; 20 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/11.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/11/head:pull/11

PR: https://git.openjdk.java.net/jfx11u/pull/11