Reset |m_touchPressed| to false in EventHandler::clear()

The member variable |m_touchPressed| in EventHandler is true if there
is at least one active touchpoint. This should be reset to false
in EventHandler::clear(), otherwise mousemove events may be
incorrectly treated as ReadOnly in handleMouseMoveEvent(). See
http://crbug.com/262794.

BUG=262794

Review URL: https://chromiumcodereview.appspot.com/22334006

git-svn-id: svn://svn.chromium.org/blink/trunk@155789 bbb929c8-8fbe-4397-9dbb-9b2b20218538