Timeline: turn TimelineFrameController into TimelineFrameModel

Keep frames in the model rather than in the TimelinePresentationModel,
since they're only present if TFM was created.

BUG=
R=loislo@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/137233014

git-svn-id: svn://svn.chromium.org/blink/trunk@166125 bbb929c8-8fbe-4397-9dbb-9b2b20218538