Inline CharacterData::dispatchModifiedEvent into the one caller

This method was only used in one location so we can inline it there. Nothing else
should ever be attempting to fire mutation events anyway.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/21012008

git-svn-id: svn://svn.chromium.org/blink/trunk@155143 bbb929c8-8fbe-4397-9dbb-9b2b20218538