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1. Make sure located events go to the top container when it's revealed.
This is accomplished with an EventRewriter which clears the target
window, which had been provided by the window service, from the event.
Without this, clicks on the top container are targeted to the
web contents window because the Window Service doesn't know about the
top container's window. In the long term, we may be able to remove this
EventRewriter with a more general solution (e.g. improving how the window
service targets the event in the first place).

2. Instead of mirroring the top container's layers, make the widget
completely transparent except for the ash-provided window controls.
Mirroring doesn't update the reveal widget in response to changes in
the top container's layers, such as when a button is hovered and an ink
drop is added.

3. Allow the reveal widget to accept events so that it's possible to
interact with the window controls. Resize the reveal widget to be just
big enough for the window controls so that clicks anywhere else will
go straight through to the underlying top container view.

4. Make the browser-side non client frame view paint the frame bg. This
is necessary because the reveal widget (i.e. window controls) is now on
top of the container view and has no client-side rendering (what the
mirroring view used to provide).

Future issues to address:
- the cursor is not updated (e.g. when hovering the omnibox) presumably
  because the cursor is coming from the window that the WS thinks is
  underneath the pointer, i.e. the web contents.

Alternatives considered:
- update the mirrored layers every time the layer tree changes. This
  doesn't seem to be easy to accomplish or likely to be very performant.
- move the top container into the reveal widget. This somewhat works
  with a few tweaks here and there to code that assumes the top container
  is always in the browser widget, but ultimately it seems that making
  the reveal widget focusable causes difficulties with window activation
  and focus traversal.

Bug: 640365
Change-Id: I03956c07354e20dadc2919f9a0402ce1f7b4a672
Reviewed-on: https://chromium-review.googlesource.com/1171547
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585649}