Timeline: split frame model into trace-specific and record-specific parts

Just moving things around, no actualy logic change, though dropped unused
FrameAdded event.

BUG=361045

Review URL: https://codereview.chromium.org/320953002

git-svn-id: svn://svn.chromium.org/blink/trunk@175881 bbb929c8-8fbe-4397-9dbb-9b2b20218538