Revert 168156 "Add an accessibility role for the iframe element."

Caused various iframe accessibility tests to fail across platforms.

> Add an accessibility role for the iframe element.
> 
> We can add a layout test after adding TestRunner support on the Chromium side,
> or we could just test it in Chromium.
> 
> BUG=347464
> 
> Review URL: https://codereview.chromium.org/183183005

TBR=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/185003006

git-svn-id: svn://svn.chromium.org/blink/trunk@168166 bbb929c8-8fbe-4397-9dbb-9b2b20218538