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This change removes the infrastructure around the WebVr displayactivate
events, including XRRuntimeEventListener methods and XRRuntime's
SetListeningForActivate for notifications to the browser process.
This also remvoes the VRDisplayClient, and some compatibility methods
on the VRService.
Future work will remove GetImmersiveVRDisplayInfo, which should be the
last remaining WebVr-only method in the mojom files.

Bug: 1017872
Change-Id: Ic149384917a25d6db4fceef35091f28350025f90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898506Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713693}