Histogram how often IDENT_SRC_URL is used.

We believe that IE7 and above no longer support user:pass@host inside urls.
We shouldn't either, but first get data on how often this is hit.
Review URL: http://codereview.chromium.org/8430022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@108150 0039d316-1c4b-4281-b951-d872f2087c98

net/http/http_auth_controller.cc

@@ -465,6 +465,7 @@ bool HttpAuthController::SelectNextAuthIdentityToTry() {
     embedded_identity_used_ = true;
     // TODO(eroman): If the password is blank, should we also try combining
     // with a password from the cache?
+    UMA_HISTOGRAM_BOOLEAN("net.HttpIdentSrcURL", true);
     return true;
   }