RFR: 8262276: Debug build of WebKit fails

[Arun Joseph]

Fixing the Debug build of WebKit.

Test: Build JavaFX using `-PCOMPILE_WEBKIT=true -PCONF=DebugNative` and test using a simple HelloWebView app.

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Commit messages:
 - 8262276: Debug build of WebKit fails

Changes: https://git.openjdk.java.net/jfx/pull/417/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=417&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262276
  Stats: 59 lines in 7 files changed: 39 ins; 16 del; 4 mod
  Patch: https://git.openjdk.java.net/jfx/pull/417.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/417/head:pull/417

PR: https://git.openjdk.java.net/jfx/pull/417