FlameChart: do not jump to the sources if the user has dragged the canvas.

BUG=none
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/23927003

git-svn-id: svn://svn.chromium.org/blink/trunk@157204 bbb929c8-8fbe-4397-9dbb-9b2b20218538