Timeline: add preview of painted picture for Paint event

Timeline: add preview of painted picture for Paint event

- enable "Capture picture" check-box in Timeline capture filter when
    Timeline on trace events experiment is enabled;
- add disabled-by-default-devtools.timeline.{picture,layers} categories
    to trace filter when the above is checked;
- associate recorded SkPicture with the last paint event of the layer
    with matching id;
- display pictures preview in Timeline's details view, when these are
    available.

BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176196

[yurys, 2014-06-16 07:34:22]

lgtm

[caseq, 2014-06-16 08:39:45]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2014-06-16 08:40:04]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/caseq@chromium.org/328143003/1

[commit-bot: I haz the power, 2014-06-16 09:42:28]

Change committed as 176196