Timeline: associate captured pictures with layer paints only

We used to attach captured pictures to last paint event, despite it
could be a FrameView, not layer paint event. In such cases, the parent
frame's layer picture would be associated with subframe paint. We should
only do this for paints that actually paint layers.

Review URL: https://codereview.chromium.org/353383002

git-svn-id: svn://svn.chromium.org/blink/trunk@177219 bbb929c8-8fbe-4397-9dbb-9b2b20218538