Fix wrong offset calculation in DocumentMarkerController.

In DocumentMarkerController::setMarkersActive(), a marker should be
skipped if its endOffset() is less than or equal to the startOffset
argument. That is because the endOffset() essentially points behind the
range of the marker, i.e. ranges are defined as [start,end).

BUG=142071

Review URL: https://chromiumcodereview.appspot.com/22924003

git-svn-id: svn://svn.chromium.org/blink/trunk@156324 bbb929c8-8fbe-4397-9dbb-9b2b20218538