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This adds an histogram to the HTTP response header parser.  I reviewed
the code, and I've convinced myself that this captures the two main
cases of HTTP header parsing (tunneling, and normal HTTP), and that
double-calls aren't occuring.  I liked having all the code in the one
translation unit.  But, I might have missed one, and we might still
be getting called twice for the same load: so the statistics might be off.

Alternatively, I could put a static function on HttpResponseHeader, to
be called from the site of parse.  But this might miss a parse.

All in all, I wanted to be sure to catch all distinct response codes
we get, so we can be sure not to stomp on anything.  So I chose the
approach using the HttpResponseHeaders constructor.

TEST=none
BUG=70428

Review URL: http://codereview.chromium.org/6317011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@72801 0039d316-1c4b-4281-b951-d872f2087c98