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There exist two interface through which a views::View is stitched to
a content::WebContentsView
- the ui::Layer is attached via a cr_setParentUiLayer method
- accessibility is attached via the accesibilityHostable protocol

Lifetimes of the embedding are slightly unclear because the attached
WebContentsView may be destroyed the NativeViewHost detaches it.

Replace the AccessibilityHostable protocol with a ViewsHostable
protocol, and add a C++ interface, ui::ViewsHostableView, which provides
a single place through which a views::View and a content::WebContentsView
are stitched together
- have content::WebContentsViewMac implement ui::ViewsHostableView
- have views::NativeWidgetHost implement ui::ViewsHostableView::Host

Use this explicit interface to ensure that the views::NativeWidgetHost
detaches from the content::WebContentsViewMac before it is destroyed.

Additional parameters and method will be added to the
ui::ViewsHostableView interface to enable stitching together
out-of-process NSViews.

R=lgrey
TBR=avi (content/ OWNERship)

Bug: 821561
Change-Id: Iddc46278cd06b9e27d3f12aa5a9e830273f3b562
Reviewed-on: https://chromium-review.googlesource.com/1252641
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595521}