Clear selected flame chart entry when overview window is being dragged

BUG=361045
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/305963006

git-svn-id: svn://svn.chromium.org/blink/trunk@175111 bbb929c8-8fbe-4397-9dbb-9b2b20218538