Touch adjustment: don't treat iframes as clickable by default

Iframe elements are hard-coded to be focusable, but unlike other
focusable elements (like text fields) this is almost always invisible
to the user.  This disables the 'is focusable' check for the special
case of iframes.  Iframes can still be candidates for adjustment for
other reasons (eg. event handlers), as can their contents of course.

BUG=405999

Review URL: https://codereview.chromium.org/495163002

git-svn-id: svn://svn.chromium.org/blink/trunk@180778 bbb929c8-8fbe-4397-9dbb-9b2b20218538