Refactoring: Add toHTMLFrameSetElement, and use it.

We'd like to avoid bare static_cast.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23702009

git-svn-id: svn://svn.chromium.org/blink/trunk@156911 bbb929c8-8fbe-4397-9dbb-9b2b20218538