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This CL implements LatencyInfo mojom types, and associated
StructTraits for serialization and deserialization. This will be used
by cc::CompositorFrameMetadata and ultimately cc::CompositorFrame
serialization/deserialization over mojo.

BUG=611802

Review-Url: https://codereview.chromium.org/2040733002
Cr-Commit-Position: refs/heads/master@{#398211}