Rename toFrameOwnerElement() to toHTMLFrameOwnerElement() for consistency.

Casting function names should be toClassName.
Also, we should use toHTMLFrameOwnerElement() to avoid static_cast<>.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23256003

git-svn-id: svn://svn.chromium.org/blink/trunk@156529 bbb929c8-8fbe-4397-9dbb-9b2b20218538