Reset startSelect and startDrag flags in EventHandler::clear.

The EventHandler::clear method does not clear the start select and
start drag fields (m_mouseDownMayStartSelect, m_mouseDownMayStartDrag)
which may cause mouse move events to be ignored across navigation.

BUG=307623
R=ojan@chromium.org, ojan@chromioum.org
TEST=diting/selection/caret-after-removing-node.html followed by
     fast/events/iframe-onmousemove.html

Review URL: https://codereview.chromium.org/27236008

git-svn-id: svn://svn.chromium.org/blink/trunk@159696 bbb929c8-8fbe-4397-9dbb-9b2b20218538