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Improves the support for requestSession in WebXR mainly focused on
allowing ARCoreDevice to receive a call when the session request
is made.

Also adds a check to both supportSession and requestSession to ensure
the device supports AR capabilities when the hit-test flag is enabled.

Bug: 835037

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;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr
Change-Id: I8090f64a981114b130e446025ec6299fbb5619e4
Reviewed-on: https://chromium-review.googlesource.com/1046107
Commit-Queue: Iker Jamardo <ijamardo@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Biao She <bshe@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562044}