Timeline: show painted pictures on layer view in Timeline

- gather paint events as part of Frame data in TimelineFrameModel;
- load pictures for paint events in TimelineLayerTreeView;
- change Layers3DView to operate over PaintProfilerSnapshots, not
    image URLs, so it may later control bitmap sizes depending on zoom;
- (drive by) fix viewport rendering in Layers3DView.

BUG=

Review URL: https://codereview.chromium.org/369913002

git-svn-id: svn://svn.chromium.org/blink/trunk@177588 bbb929c8-8fbe-4397-9dbb-9b2b20218538