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This adds a few helper functions to WidgetTest for getting information
from a native widget and makes widget_test.h responsible for defining
typedefs for PlatformNativeWidget and (when not ChromeOS)
PlatformDesktopNativeWidget.

4 aura-specific tests (that are #ifdefd out on ChromeOS) that use
DesktopAuraTopLevelWindowTest are moved from widget/widget_unittest.cc
to widget/desktop_aura/desktop_native_widget_aura_unittest.cc

widget_unittests.cc doesn't yet compile on Mac with this alone due to
the event generator (addressed in CL/322893005).

BUG=378134
TEST=views_unittests

Review URL: https://codereview.chromium.org/334573008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278400 0039d316-1c4b-4281-b951-d872f2087c98