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
Add layout test for access to naked global object

BUG=324812
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/99153004

git-svn-id: svn://svn.chromium.org/blink/trunk@163075 bbb929c8-8fbe-4397-9dbb-9b2b20218538