Use view() instead of toRenderView(document()->renderer()).

While equivalent, the former is much more readable than the latter.

Also add a FIXME to replace this by something better if possible, with a reference to the associated bug.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23217002

git-svn-id: svn://svn.chromium.org/blink/trunk@156186 bbb929c8-8fbe-4397-9dbb-9b2b20218538