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=

Review URL: https://codereview.chromium.org/328143003

git-svn-id: svn://svn.chromium.org/blink/trunk@176196 bbb929c8-8fbe-4397-9dbb-9b2b20218538