Prevent synthetic mouse moves from firing during trackpad scroll.

Prevent synthetic mouse moves from firing during trackpad scroll.

Previously we dispatched synthetic mouse move events during trackpad
and mousewheel scrolling. These events were throttled, but still
caused significant jank on some pages.

This patch prevents these synthetic mouse moves from being dispatched
until the scroll is complete.

Perf tryjob here: 
https://codereview.chromium.org/1213113003
Results here:
http://storage.googleapis.com/chromium-telemetry/html-results/results-2015-06-29_10-43-42

BUG=317007
TEST=smoothness.tough_scrolling_cases/responsive-biggest_jank_thread_time

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=198053

[tdresser, 2015-06-29 17:49:06]

tdresser@chromium.org changed reviewers:
+ rbyers@chromium.org

[tdresser, 2015-06-29 17:49:07]

The performance test is behaving correctly.
http://storage.googleapis.com/chromium-telemetry/html-results/results-2015-06...

Monitoring is on for:
tasks_per_frame_raster, thread_total_all_cpu_time_per_frame,
thread_raster_cpu_time_per_frame

[Rick Byers, 2015-06-29 18:42:39]

Excellent, LGTM!

Any chance we have an UMA metric we may see the benefits of this in?  Eg. total
CPU usage / power consumption?

[tdresser, 2015-06-29 18:54:12]

On 2015/06/29 18:42:39, Rick Byers wrote:
> Excellent, LGTM!
> 
> Any chance we have an UMA metric we may see the benefits of this in?  Eg.
total
> CPU usage / power consumption?

Hard to say.

thread_total_all_cpu_time_per_frame is noisy enough that it's hard to say if
we'll see an improvement in the wild. I think it depends a lot on how frequently
this issue shows up in the wild.

[tdresser, 2015-06-30 11:54:19]

The CQ bit was checked by tdresser@chromium.org

[commit-bot: I haz the power, 2015-06-30 11:54:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1157303006/20001

[commit-bot: I haz the power, 2015-06-30 11:57:44]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=198053