Fix gradual deterioration of cadence based rendering.

Fix gradual deterioration of cadence based rendering.

When a cadence based rendering sequence exceeds drift, we should not
count the over-rendered frame against the next in sequence.  Doing
so causes an accumulation of errors.

Also extends the moving average for durations to 32 values and the
allowable drift to 16.66ms to stabilize the Catzilla 60fps and
Elite: Dangerous 60fps clips, which were hitting hysteresis very
frequently.

BUG=439548
TEST=new unittest.

Committed: https://crrev.com/f0ecec8eb2f9a1b8937d918f233b663ffc26bc01
Cr-Commit-Position: refs/heads/master@{#329987}

[DaleCurtis, 2015-05-14 22:30:30]

dalecurtis@chromium.org changed reviewers:
+ xhwang@chromium.org

[DaleCurtis, 2015-05-14 22:30:30]

Polishes the experience for 60fps clips and fixes a bug with cadence based
rendering.

[xhwang, 2015-05-14 23:00:14]

lgtm

[DaleCurtis, 2015-05-14 23:07:08]

The CQ bit was checked by dalecurtis@chromium.org

[commit-bot: I haz the power, 2015-05-14 23:07:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1139973003/1

[commit-bot: I haz the power, 2015-05-15 00:11:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-15 00:12:35]

Patchset 1 (id:??) landed as
https://crrev.com/f0ecec8eb2f9a1b8937d918f233b663ffc26bc01
Cr-Commit-Position: refs/heads/master@{#329987}