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Add build flags for async VR.
Add proguard configs for keeping VR compatible with base.

Bug: 947544
Change-Id: Ibbfc1eb9325d09e6ceb3e241bc82ef4ff0ad1f1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1552020Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Anthony Cui <cuianthony@google.com>
Cr-Commit-Position: refs/heads/master@{#649670}