MacViews: Fix NativeWidgetMac's GetTopLevelWidget()

This function never returned a top level widget; it instead returned
the root widget. Fix the underlying GetTopLevelNativeWidget()
function so that it returns the correct widget.

This is a relanding of https://crrev.com/c/1982682, which was reverted
due to code that relied on GetTopLevelNativeWidget()'s misbehavior.
Now that the code is fixed in https://crrev.com/c/2792171, it is safe
to reland this fix.

Bug: 957362
Change-Id: Ie025edc06f8d6bce7c571fbdf551fa3336c95869
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2795421
Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#868249}