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This creates a policy engine that forwards calculated priority values
out of the graph, applying the priorities to RenderProcessHosts (and
subsequently to the process itself).

BUG=971272

Change-Id: I11c8c040e5382a8fa6881c40f80f6885f26e88df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904367
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713859}