TimelineGrid: move itemsGraphElement from TimelineGrid to TimelineView where it is actually using.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/194713002

git-svn-id: svn://svn.chromium.org/blink/trunk@169092 bbb929c8-8fbe-4397-9dbb-9b2b20218538