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The delegates communicate through 2 channels in the android
implementation:
1. WebXrPresentationState, owned by the scheduler.
2. The public interface of VrShellGl. Ideally, this interface should
remain very small to ensure the task separation.

VrShellGl now only implements the CompositorDelegate, and it's soon to
be renamed. RenderLoop now owns the delegates.

Bug: 875291
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:linux_vr;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ia30e62ed249b5fc0682b8f2b943eef615408daa3
Reviewed-on: https://chromium-review.googlesource.com/1191864
Commit-Queue: Aldo Culquicondor <acondor@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589342}