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Take the existing BridgedNativeWidgetHost and break it into two parts
- a mojo interface views_bridge_mac::mojo::BridgedNativeWidgetHost
- a helper and "stuff we haven't mojo-ified-yet" interface
  BridgedNativeWidgetHostHelper
  - this is not named BridgedNativeWidgetHost to avoid conflicts
  - this will be moved to //ui/views_bridge_mac soon (build is tangled)

Add mojo support for ui::DialogButton, because it's used by this
interface.

For keyboard event handling, use the BridgedNativeWidgetHostHelper
interface because a mojo-compatible interface would be hard for the
callers to use.

Bug: 859152
Change-Id: If6802ab4d6e518184cd3fa57eef5d8a510b80e59
Reviewed-on: https://chromium-review.googlesource.com/1198014Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588144}