Remove support for the obsolete <isindex> tag.

This patch removes all special-casing for the <isindex> tag; it
now behaves exactly like <foo> in all respects. This additionally
means that we can remove the special-casing for forms containing
<input name="isindex"> as their first element.

The various tests for <isindex> have been deleted, with the
exception of the imported HTML5Lib tests. It's not clear that
we should send them patches to remove the <isindex> tests, at
least not while the element is (an obsolete) part of HTML5, and
supported by other vendors.

I've just landed failing test results here. That seems like
the right thing to do.

"Intent to Remove" discussion: https://groups.google.com/a/chromium.org/d/msg/blink-dev/14q_I06gwg8/0a3JI0kjbC0J

Review URL: https://codereview.chromium.org/96653004

git-svn-id: svn://svn.chromium.org/blink/trunk@168010 bbb929c8-8fbe-4397-9dbb-9b2b20218538