RFR: 8286831: WebView can't switch to Chinese input method in GTK3

[pe-3-6n]

This PR fixes a bug where the can't switch input method in GTK3.

-------------

Commit messages:
 - 8286831: Fixed so that input method can be switched in GTK3

Changes: https://git.openjdk.org/jfx/pull/1660/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1660&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8286831
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1660.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1660/head:pull/1660

PR: https://git.openjdk.org/jfx/pull/1660