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In the future, we'd like to merge Surface client ID registration and
SurfaceFactory creation. SurfaceFactory is tied to an OutputSurface. Thus, we
cannot register a namespace hierarchy relationship until after we create an
OutputSurface for the ui::Compositor. However, we create a new OutputSurface
every time the GPU process is restarted. Thus, we need a mechanism to store
these relationships between clients across GPU restarts.

This is particularly important if the display compositor lives in the same
process GPU and these relationships would be lost. This CL stores the
relationship between DelegatedFrameHost and ui::Compositor in ui::Compositor.

Note that depending on the final shape of the display compositor host API,
we may wish to centralize all hierarchy relationship registration including
OOPIF.

BUG=629940
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2175293003
Cr-Commit-Position: refs/heads/master@{#407770}