Drop isindex from HTMLTagNames.

Followup from https://codereview.chromium.org/96653004/

This requires an update to the generation scripts, as they must treat
the fallback interface as an interface supporting multiple tag names.
This only accidentally worked, as both 'image' and 'isindex' were
listed as using the HTMLUnknownElement interface. Removing 'isindex'
exposed the bug.

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

Review URL: https://codereview.chromium.org/182843002

git-svn-id: svn://svn.chromium.org/blink/trunk@168033 bbb929c8-8fbe-4397-9dbb-9b2b20218538