RFR: 8322703: Intermittent crash in WebView in a JFXPanel from IME calls on macOS [v2]

[Kevin Rushforth]

> As noted in the JBS bug, this is a follow-on to [JDK-8196011](https://bugs.openjdk.org/browse/JDK-8196011) that I discovered while testing the fix for [JDK-8221261](https://bugs.openjdk.org/browse/JDK-8221261) (a deadlock in the IME code when using WebView in a JFXPanel on macOS).
> 
> I have tested this in connection with with the proposed fix for JDK-8221261, although it is a valid fix regardless.
> 
> This expands the fix done in [JDK-8221261](https://bugs.openjdk.org/browse/JDK-8221261) to call all of the WebKit methods on the right thread. Additionally, we sometimes see spurious exceptions where the committed text is coming back as null, so I changed the log level to "fine" rather than "severe" for those exceptions. I'll file a follow-up bug  to see if any of these are real problems or not.

Kevin Rushforth has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/master' into 8322703-webkit-ime-crash
 - 8322703: Intermittent crash in WebView in a JFXPanel from IME calls on macOS

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1321/files
  - new: https://git.openjdk.org/jfx/pull/1321/files/bd5a05fa..c5ce4a7f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1321&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1321&range=00-01

  Stats: 55 lines in 8 files changed: 44 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jfx/pull/1321.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1321/head:pull/1321

PR: https://git.openjdk.org/jfx/pull/1321