Use tighter typing for Document::ownerElement() return value

Use tighter typing for Document::ownerElement() return value. It returns an
HTMLFrameOwnerElement* but a lot of callers assign it to an Element* variable.

This gets rid of a few unnecessary type checks and avoids an unnecessary
isHTMLElement() check in isHTMLFrameElementBase() in several cases.

R=adamk@chromium.org, rob.buis@samsung.com

Review URL: https://codereview.chromium.org/476223003

git-svn-id: svn://svn.chromium.org/blink/trunk@180400 bbb929c8-8fbe-4397-9dbb-9b2b20218538