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The logic to track surface references for surface embeddings was built
as part of FrameGenerator. Generalize the logic and remove from
FrameGenerator, creating the class EmbeddedSurfaceTracker.

EmbeddedSurfaceTracker keeps track of surface references from one client
surface to many embedded surfaces. It also handles generating new
references when the client surface changes. Includes unit tests.

This simplifies FrameGenerator and should make it easier to use surface
references in other clients.

BUG=659227
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2610063007
Cr-Commit-Position: refs/heads/master@{#442997}