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Currently GRC's process CPU measurement mechanisms are tightly
integrated with the process coordination unit implementation.
This CL moves the CPU usage measurement from the resource_coordinator
service to the content_browser service.

Decoupling process CPU measurement from the CU model allows the
coordination units to focus on representing browser state within GRC,
rather than updating it. In GRC terms, the migrated render process CPU
measurement infrastructure is a "probe" that externally triggers
updates to the CU graph.

A new feature flag has been added for enabling render process
CPU usage profiling. To do so, run:

out/Default/chrome 
  --enable-features=GlobalResourceCoordinator,GRCRenderProcessCPUProfiling \
  http://example.com

BUG=691886,741768

Change-Id: I2fc7609c6963776a167c5f76abe078dedb0e2efe
Reviewed-on: https://chromium-review.googlesource.com/565906
Commit-Queue: Matthew Halpern <matthalp@google.com>
Reviewed-by: Zhen Wang <zhenw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Oystein Eftevaag <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488352}