[jfx11u] RFR: 8262276: Debug build of WebKit fails

[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:
 - 8262276: Debug build of WebKit fails

Changes: https://git.openjdk.java.net/jfx11u/pull/13/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=13&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262276
  Stats: 34 lines in 6 files changed: 21 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/13.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/13/head:pull/13

PR: https://git.openjdk.java.net/jfx11u/pull/13