FlameChart: stop dragging on the chart borders.

It was regressed at r167927 DevTools: implement zoom and pan in timeline FlameChart mode.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/179163008

git-svn-id: svn://svn.chromium.org/blink/trunk@167988 bbb929c8-8fbe-4397-9dbb-9b2b20218538