Add first_gesture_scroll_update_latency metric

Add first_gesture_scroll_update_latency metric

Add a metric for measuring the time it takes to process the very first
gesture scroll update input event. The first scroll gesture can often
get delayed by work related to page loading, which is what we hope to
improve with the Blink scheduler. This metric will help us track the
effectiveness of those improvements.

Original patch from Simon Pick <picksi@chromium.org>;

BUG=391005
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=290004

[Sami, 2014-08-13 18:06:38]

Since Simon is OoO and I couldn't hijack
https://codereview.chromium.org/397663002/, I've taken his patch and rewritten
it against ToT. PTAL.

[jdduke (slow), 2014-08-14 14:55:52]

On 2014/08/13 18:06:38, Sami wrote:
> Since Simon is OoO and I couldn't hijack
> https://codereview.chromium.org/397663002/, I've taken his patch and rewritten
> it against ToT. PTAL.

Echoing my non-owner LGTM :).  nduca@, what do you think?

[Sami, 2014-08-15 15:23:03]

Thanks Jared. Since this gives the same results as Simon's earlier patch, I'm
going to land this now and beg for forgiveness later. This way we can also
establish a baseline before https://codereview.chromium.org/363383002/ goes in.

[Sami, 2014-08-15 15:23:11]

The CQ bit was checked by skyostil@chromium.org

[commit-bot: I haz the power, 2014-08-15 15:25:26]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/skyostil@chromium.org/467343002/1

[commit-bot: I haz the power, 2014-08-15 20:48:56]

Committed patchset #1 (1) as 290004