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This is a step toward supporting shortcut keys and menu interface items
in PWAs. This breaks the function -[BrowserWindowCommandHandler
validateUserInterfaceItem] into two parts
- The part to look up the browser command and to update the interface
  item still remains in BrowserWindowCommandHandler (which will be in
  the in the PWA process).
- The part to access the Browser*, which will be in the browser process,
  which is
  - routed through mojo to the BridgedNativeWidgetHostImpl
  - to the NativeWidgetMac
  - which is subclassed as BrowserFrameMac
  - which calls BrowserWindowCommandHandlerValidateUserInterfaceItem
    to implemented the parts that used to be in
    BrowserWindowCommandHandler

Also add required interface to look up a BridgedNativeWidgetImpl from
an NSWindow.

Bug: 895169
Change-Id: Ie81a1e5a44537b07183d4c29b89d2da1951137c0
Reviewed-on: https://chromium-review.googlesource.com/c/1357829
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613255}