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

Source/core/html/HTMLImageFallbackHelper.h

Index: Source/core/html/HTMLImageFallbackHelper.h
diff --git a/Source/core/html/HTMLImageFallbackHelper.h b/Source/core/html/HTMLImageFallbackHelper.h
index 27413d1353017b37360087e256b90b77ac6d2b65..9aebbfd622f6c629cf122e4a79c73856598e1e45 100644
--- a/Source/core/html/HTMLImageFallbackHelper.h
+++ b/Source/core/html/HTMLImageFallbackHelper.h
@@ -5,6 +5,7 @@
 #ifndef HTMLImageFallbackHelper_h
 #define HTMLImageFallbackHelper_h
 
+#include "wtf/Allocator.h"
 #include "wtf/PassRefPtr.h"
 
 namespace blink {
@@ -13,6 +14,7 @@ class Element;
 class ComputedStyle;
 
 class HTMLImageFallbackHelper {
+    STATIC_ONLY(HTMLImageFallbackHelper);
 public:
     static void createAltTextShadowTree(Element&);
     static PassRefPtr<ComputedStyle> customStyleForAltText(Element&, PassRefPtr<ComputedStyle> newStyle);