RFR: 8301219: JavaFX crash when closing with the escape key [v3]

[Martin Fox]

> While processing a key down event the Glass GTK code sends out PRESSED and TYPED KeyEvents back to back. If the stage is closed during the PRESSED event the code will end up referencing freed memory while sending out the TYPED event. This can lead to intermittent crashes.
> 
> In GlassApplication.cpp the EventCounterHelper object ensures the WindowContext isn't deleted while processing an event. Currently the helper object is being created *after* IME handling instead of before. If the IME is enabled it's possible for the WindowContext to be deleted in the middle of executing a number of keyboard-related events.
> 
> The fix is simple; instantiate the EventCounterHelper object earlier. There isn't always a WindowContext so I tweaked the EventCounterHelper to do nothing if the context is null.
> 
> To make the crash more reproducible I altered the WindowContext such that when it's deleted the freed memory is filled with 0xCC. This made the crash more reproducible and allowed me to test the fix. I did the same with GlassView since that's the only other Glass GTK class that's instantiated with `new` and discarded with `delete`.

Martin Fox has updated the pull request incrementally with one additional commit since the last revision:

  Consistent use of FILL in mem debug code.

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1307/files
  - new: https://git.openjdk.org/jfx/pull/1307/files/6a4a4e63..09f8ede5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1307&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1307&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1307.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1307/head:pull/1307

PR: https://git.openjdk.org/jfx/pull/1307