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This CL converts XRCompositorHostPtr and Binding<XRCompositorHost>
in chrome and device to the new Mojo type, and use
pending_remote<XRCompositorHost> in isolated_xr_service.mojom.

Bug: 955171
Change-Id: Ib5d6f46f104b525806ef4363b944dc0c079917e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1847451
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#704646}