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This CL adds CPU usage graph to the Ash HUD overlay:
  Gray - Idle
  Blue - User
  Red - System
  Magenta - Other

Internally this Cl adds support for switching between graphs (CPU and RAM).

Bug: 1085713, 1101715
Change-Id: I6ea1a07d77122c160ddb2229eb6ce5eb4cc829bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2355553Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799566}