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New functions have been added so a sync token can represent
multiple resources at once. Adding a single sync token at
the end of generating a multitude of resources can be a win.

The most obvious use cases of this is the video layer which
generates a resource for each layer. A single sync token is
now generated at the end of the layer generation.

The layer tree UI resource also generates its own sync token.

BUG=584381
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1707143002

Cr-Commit-Position: refs/heads/master@{#376039}