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This fixes a bug where pressing tab would move focus forward by two UI
elements.

There was a workaround in PlatformDisplayDefault that would synthesize a
duplicate ET_KEY_PRESSED for every key down in order to emulate Windows
WM_CHAR synthesis. See https://codereview.chromium.org/1234623004
The workaround isn't needed on mash, so I removed it.

I added a unit test, which required some small changes to
PlatformDisplayDefault and OzonePlatform.

Allowed the ui::ImageCursors parameter to be null because the test harness
does not provide the resource bundle needed for cursors.

Revert a workaround in InputMethodBridge for these synthesized events,
see https://codereview.chromium.org/2576973003/

BUG=706574,707055
TEST=added to mus_ws_unittests

Review-Url: https://codereview.chromium.org/2795503002
Cr-Commit-Position: refs/heads/master@{#463036}