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With this patch, both Windows and Ozone implementations use PlatformWindow for
interacting with the native windowing system. Eventually, the X11 implementations
will also use this, and WindowTreeHost will have a single implementation on all
platforms.

BUG=none
R=ben@chromium.org

Previously landed in r284850, but reverted in r284905 because it broke a multi-
window ash unit-test on Windows. The test is now disabled (r284931).

Review URL: https://codereview.chromium.org/400413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284986 0039d316-1c4b-4281-b951-d872f2087c98