TimelinePanel: a fix for Frames mode overview. It kept old records.

Frame mode overview has its own array of frames. We have to clear it
when we start new record session.

BUG=
R=alph@chromium.org

Review URL: https://codereview.chromium.org/136683002

git-svn-id: svn://svn.chromium.org/blink/trunk@164965 bbb929c8-8fbe-4397-9dbb-9b2b20218538