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After this change, ContentElement should only be used for the main
web content. PlatformUiElement can be used to host platform UIs (such
as View on Android).


Bug: NONE
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: If068d52d3134b5e1f70392523fdbfa4c047d1081
Reviewed-on: https://chromium-review.googlesource.com/1049961
Commit-Queue: Biao She <bshe@chromium.org>
Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557177}