Revert "Introduce toHTMLFrameElement" (r154387).

Causing asserts on many layout tests on Linux Debug, e.g., accessibility/scroll-to-global-point-iframe-nested.html: STDERR: ASSERTION FAILED: !node || node->hasTagName(HTMLNames::frameTag)

TBR=haraken

Review URL: https://codereview.chromium.org/19621002

git-svn-id: svn://svn.chromium.org/blink/trunk@154404 bbb929c8-8fbe-4397-9dbb-9b2b20218538