DevTools: Fix timeline overview window selection

The window selection is shown in a left corner when drag was initiated on a marker.

R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/211343003

git-svn-id: svn://svn.chromium.org/blink/trunk@169954 bbb929c8-8fbe-4397-9dbb-9b2b20218538