Added UseCounter for XML external DTD.

Source/core/frame/UseCounter.h

Index: Source/core/frame/UseCounter.h
diff --git a/Source/core/frame/UseCounter.h b/Source/core/frame/UseCounter.h
index 515f3f6b60377ec46d56d884f7f56e7db88f767e..4b2be9e49abe8c9596481ec30e3044e5eb900e52 100644
--- a/Source/core/frame/UseCounter.h
+++ b/Source/core/frame/UseCounter.h
@@ -495,6 +495,8 @@ public:
         SrcsetWDescriptor = 524,
         SelectionContainsNode = 525,
         MediaStreamEnded = 526,
+        XMLExternalDTD = 527,

[kouhei (in TOK), 2014/09/08 17:07:58]

openFunc is also used from xinclude and schema. Schema loading is disabled in
Blink, but xinclude may be not.

Let's use a generic name here, like XMLExternalResourceLoad.

[tasak, 2014/09/09 03:07:06]

I see.
Done.

+
         // Add new features immediately above this line. Don't change assigned
         // numbers of any item, and don't reuse removed slots.
         // Also, run update_use_counter_feature_enum.py in chromium/src/tools/metrics/histograms/