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This CL converts AppWindow in app_window.mojom using
PendingReceiver, Remote, and ReceiverSet.

Bug: 955171
Change-Id: I254824287a2dc520a8b1383bb34e9a829c7a9942
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846637Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Julie Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#703678}