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`.

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

Commit messages:
 - Minor cleanup.
 - Version of test that succeeds on Windows
 - WindowContext is no longer deleted in the middle of processing a key event

Changes: https://git.openjdk.org/jfx/pull/1307/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1307&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301219
  Stats: 197 lines in 5 files changed: 188 ins; 1 del; 8 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

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