aura: Remove WindowTreeHost::PostNativeEvent().
WindowTreeHost::PostNativeEvent() is only used in tests. So remove this from
the public API. Instead:
. For Win32, directly use PostMessage (this is used in only two places).
. For Ozone, use EventSourceTestApi to inject the event (this is equivalent to
current code, in terms of not doing the right thing when menus are open).
. For X11, introduce aura::test::PostEventToWindowTreeHost() instead, and use
it in tests.
BUG=none
R=sky@chromium.org
Review URL: https://codereview.chromium.org/
710553002
Cr-Commit-Position: refs/heads/master@{#303315}