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This gets 3 views_unittests passing:
 - ViewTargeterTest.HitTestCallsOnView,
 - ViewTest.CanProcessEventsWithinSubtree,
 - ViewTest.GetEventHandlerForRect.

These were failing because the size of the RootView was being reset to
the Widget size when first added. This happened because NativeWidgetMac
was not ensuring the RootView was set up properly immediately after a
call to Widget::Init() for all Widget types (i.e. those without a
NonClientView).

When there _is_ a NonClientView, Widget::Init() calls SetContentsView()
which would eventually trickle down to
NativeWidgetMac::ReorderNativeViews(), which is where NativeWidgetMac
would set up the root view.

BUG=378134

Review URL: https://codereview.chromium.org/559873002

Cr-Commit-Position: refs/heads/master@{#294296}