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On Windows, Widgets with shadow_elevation specified are currently drawn
incorrectly. This is because we spawn a native top-level window, which
ignores shadow_elevation.

This is to be expected, since the Windows window manager has no concept
of Material shadow elevation.

This CL forces those Widgets with shadow_elevation to use an Aura
non-toplevel window.

The caveat is that these widgets are now clipped to the root browser
window. (It can't be bigger.) But it has the benefits of being:

 1. Low maintainence
 2. Click-through shadow
 3. Proper hit testing

Bug: 823535, 838667
Change-Id: I008ab64d0f3b7c2811d11ac94f00d041c45bf730
Reviewed-on: https://chromium-review.googlesource.com/1028833
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555267}