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Set up a startup sampling-profiler for the compositor thread in the
renderer process. Notable changes:
. Callback to the RendererClient when the compositor-thread is created.
  The client uses this callback to set-up the sampling profiler.
. Allow the renderer processes to connect to the browser to report the
  profiler data over mojom. Add the --start-stack-profiler flag to the
  renderer's cmd-line so that it can initiate sampling profiler. Select
  every fifth renderer for collecting the profile data from, so that we
  can still get enough useful data, but don't overburden bandwidth or
  memory usage.

BUG=788808

Change-Id: I478184502a17f4dbb83f03a67a7ef482c92c0a78
Reviewed-on: https://chromium-review.googlesource.com/894848Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534235}