Remove EventHandler::handleMouseUp().

This used to call EventHandler::eventActivatedView(), which was removed
in r149444.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23600018

git-svn-id: svn://svn.chromium.org/blink/trunk@157471 bbb929c8-8fbe-4397-9dbb-9b2b20218538