dashboard: Include metric name for rendering alerts.
dashboard: Include metric name for rendering alerts.

For the rendering.desktop/rendering.mobile benchmarks, the alerts
should include the name of the metric that regressed. So instead
of

  'x% regression in rendering.deskop at X:Y'

the alert (and the associated crbug summary) will be:

  'x% regression in rendering.desktop/frame_times at X:Y'.

BUG=catapult:#4585

Change-Id: I772f7f19f1465df472a0f845513a03fbaf6b79b2
Reviewed-on: https://chromium-review.googlesource.com/1236753
Reviewed-by: Dave Tu <baa605d781958521249fdcf7bad76b01d943134a@chromium.org>
Commit-Queue: Sadrul Chowdhury <be235e35a6c144329e4aa1b0fbe2c3f8c71f3676@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult