Have RenderView's enclosingSeamlessRenderer() take a Document reference in argument

Have RenderView's enclosingSeamlessRenderer() take a Document reference in argument
instead of a pointer since the document can never be NULL. This avoids having to
convert a reference into a pointer at the call sites and gets rid of a useless
null-check.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23533013

git-svn-id: svn://svn.chromium.org/blink/trunk@157099 bbb929c8-8fbe-4397-9dbb-9b2b20218538