Count usage of the obsolete 'isindex' element.

I can imagine wanting to count other element types as well; I've
implemented this CL with that in mind, by putting the UseCounter getter
on the parser so that we can grab it from anything involved with parsing
rather than sticking it somewhere in the tree-builder.

Review URL: https://codereview.chromium.org/98743002

git-svn-id: svn://svn.chromium.org/blink/trunk@163074 bbb929c8-8fbe-4397-9dbb-9b2b20218538