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Create an origin trial for WebXR and a separate one for exposing controllers as Gamepads.
Controllers will still be exposed as Gamepads by default for WebVR origin trial, but must be
enabled separately for WebXR.

Bug: 779691

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I639b259ea2a583bdbd3f18b6d30b7e107cdb21c1
Reviewed-on: https://chromium-review.googlesource.com/982679
Commit-Queue: Anna Offenwanger <offenwanger@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548291}