Add support for ManagedMemoryPolicy (de)serialization

As part of serializing cc::LayerTreeHost, we also need to
serialize the cc::LayerTreeSettings and the
cc::ManagedMemoryPolicy.

This CL focuses on the cc::ManagedMemoryPolicy and the
gpu::MemoryAllocation::PriorityCutoff enum.

BUG=561210
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1490513005

Cr-Commit-Position: refs/heads/master@{#362603}
12 files changed