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I couldn't bring myself to add another #ifdef to
view_event_test_base.cc, so this splits out the platform-specific stuff
and paves the way to get interactive_ui_tests compiling on MacViews.

This is done by adding a ViewEventTestPlatFormPart interface, which is
implemented for (views platforms): chromeos, ash, and mac.

There was also code for a non-ash aura platform, but all build
configurations that would have used it are broken, so it's just rot and
is removed.

BUG=399191

Review URL: https://codereview.chromium.org/443553003

Cr-Commit-Position: refs/heads/master@{#288966}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288966 0039d316-1c4b-4281-b951-d872f2087c98