cc: Enable switching main thread back to low latency mode

cc: Enable switching main thread back to low latency mode

Enable the algorithm for switching the main thread back from high to low
latency mode. The algorithm was implemented earlier by dominikg@ in
https://codereview.chromium.org/54913003.

I've done a few telemetry runs to measure the impact on latency and
smoothness. The following results are from Nexus 5.

Touch latency (touch_scroll_latency):
- 25% improvement in smoothness.key_silk_cases.
- 15% improvement in smoothness.top_25.
- 4% improvement in scheduler.tough_scheduler_cases[1] (21% improvement
  on tests that specifically target main thread latency).

Jank (jank_count):
- 15% regression in smoothness.key_silk_cases (discrepancy-based jank up
  5%).
- Unchanged in smoothness.top_25 (discrepancy-based jank up 10%).
- Unchanged in tough_scheduler_cases.

The jank metrics seem to be noisy, so I was not able to measure exactly
how they were affected. Potentially the impact can be mitigated by
tuning the draw duration estimates (crbug.com/243459).

TEST=tools/perf/run_benchmark --browser=android-chrome smoothness.key_silk_cases
BUG=309648

[1] https://codereview.chromium.org/167013003/ needed to get meaningful
    results.

Review URL: https://codereview.chromium.org/169903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254828 0039d316-1c4b-4281-b951-d872f2087c98