Make classes and structures in core/html fast-allocated.

Source/core/html/parser/HTMLEntityParser.h

Index: Source/core/html/parser/HTMLEntityParser.h
diff --git a/Source/core/html/parser/HTMLEntityParser.h b/Source/core/html/parser/HTMLEntityParser.h
index 42df842aa564d2d9afc6bc43552e77dc6aa7b30d..4479c23639d64e2ed7443603142fd55b42276fec 100644
--- a/Source/core/html/parser/HTMLEntityParser.h
+++ b/Source/core/html/parser/HTMLEntityParser.h
@@ -28,10 +28,12 @@
 #define HTMLEntityParser_h
 
 #include "platform/text/SegmentedString.h"
+#include "wtf/Allocator.h"
 
 namespace blink {
 
 class DecodedHTMLEntity {
+    STACK_ALLOCATED();
 private:
     // HTML entities contain at most four UTF-16 code units.
     static const unsigned kMaxLength = 4;