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

git-svn-id: svn://svn.chromium.org/blink/trunk@168156 bbb929c8-8fbe-4397-9dbb-9b2b20218538