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The NSView hierarchy for most windows is
- BridgedNativeWidget
  - WebContentsView (*)
    - RenderWidgetHostView
This patch adds a cross-process interface for WebContentsView. This
interface created via the NSViewBridgeFactory interface, using the
new mojo method CreateWebContentsNSViewBridge.

Add the mojo interface content::mojom::WebContentsNSViewBridge and
content::mojom::WebContentsNSViewClient.
- The interface that is exposed is motivated by the native Cocoa calls
  made by views::NativeViewHostMac (which are targeting the
  WebContentsViewCocoa).
- Note that this interface is public (unlike RenderWidgetHostNSView)
  because it doesn't have any content-private dependencies.

Add methods to the C++ interface ui::ViewsHostableView (which is
implemented by WebContentsViewMac), to perform all of the functions
currently done by Cocoa calls on WebContentsViewCocoa.

Add the class content::WebContentsNSViewBridge, which implements the
new mojo interface.
- This performs the same Cocoa calls that are done in the browswer
  process.
- Note that this class does not instantiate WebContentsViewCocoa (the
  NSView sub-class used by WebContents), rather, it uses a vanilla
  NSView. This may change in the future, if need be.
- Note that this NSView is in addition to the WebContentsViewCocoa
  created in the browser process (not in place of it).

Update RenderWidgetHostMac::MigrateNSViewBridge to take the arguments
that content::WebContentsViewMac will send it.

Bug: 821651
Change-Id: Ic1dd5c469a8dab32837cdf23cdda34d24d6240cb
Reviewed-on: https://chromium-review.googlesource.com/1253095
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595751}