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UKM updates the per-WebContents source id during WebContentsObserver callbacks.
VR fires UKM events during WebContentsObserver callbacks.  Navigation events
may get fired to VR or UKM first, so we may get stale ids.

BUG=866633

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: Iaf99f70b0defecd34156d64526ecd329f2315876
Reviewed-on: https://chromium-review.googlesource.com/1147504Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577994}