TimelinePanel: fix TimelineMemoryOverview.

rename the functions 
add annotation
add missing brackets to data property

BUG=

Review URL: https://codereview.chromium.org/254993003

git-svn-id: svn://svn.chromium.org/blink/trunk@172769 bbb929c8-8fbe-4397-9dbb-9b2b20218538