RFR: 8261927: WebKit build fails with Visual Studio 2017

[Arun Joseph]

The WebKit build fails with Visual Studio 2017.

Issue: Visual Studio 2017 doesn't support if constexpr in lamda

Test: Build webkit with the VS2017 compiler with and without this fix. It should fail without the fix and build with the fix.

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Commit messages:
 - 8261927: WebKit build fails with Visual Studio 2017

Changes: https://git.openjdk.java.net/jfx/pull/405/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=405&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261927
  Stats: 33 lines in 1 file changed: 31 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/405.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/405/head:pull/405

PR: https://git.openjdk.java.net/jfx/pull/405