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Add the NSAccessibilityRemoteUIElement interface along with helper
functions to use the interface with stl structures.

Make the previously-dummy GetNativeViewAccessible implementation of
perform a sync call to BridgedNativeWidgetHost to set up the required
accessibility tree structures.
- Send to the browser process the accessibility tokens for the
  NSWindow and its content NSView.
- Retrieve the accessibility token for the root AXPlatformNodeCocoa,
  along with the browser process id.

This is not sufficient for accessibility to work. The remaining parts
are to
- Update the AXPlatformNodeCocoa class to return the object created
  from the accessibility tokens.
  - Add TODOs where this needs to be hooked up, but defer the actual
    change to a separate patch.
- Splice together the accessibility trees for content and views.

Create a ScopedAccessibilityFocus class to override the value returned
by -[NSApplication accessibilityFocusedUIElement]. This is required for
cross-process accessibility methods to find the appropriate focused
element.

Bug: 900846
Change-Id: I09df0bfddd82b7904131b195ece55dc803c56aa9
Reviewed-on: https://chromium-review.googlesource.com/c/1337377
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608690}