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This adds HostFrameSinkManager::FindRootFrameSinkId() to find the root
frame sink id. This is necessary as
window->GetHost()->compositor()->frame_sink_id() is not the root frame sink in
mash.

BUG=879791
TEST=none

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I5a1520b2ba144cb231d254059810ff153add3a49
Reviewed-on: https://chromium-review.googlesource.com/1249152
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595512}