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

Source/core/dom/custom/CustomElementProcessingStep.h

Index: Source/core/dom/custom/CustomElementProcessingStep.h
diff --git a/Source/core/dom/custom/CustomElementProcessingStep.h b/Source/core/dom/custom/CustomElementProcessingStep.h
index 35101a9678d9679806c0582f16ad7bf346bc9ff4..d84f6611dd56d594ade63ea2be0d70cbda7a7da2 100644
--- a/Source/core/dom/custom/CustomElementProcessingStep.h
+++ b/Source/core/dom/custom/CustomElementProcessingStep.h
@@ -37,6 +37,7 @@
 namespace blink {
 
 class CustomElementProcessingStep : public NoBaseWillBeGarbageCollectedFinalized<CustomElementProcessingStep> {
+    WTF_MAKE_FAST_ALLOCATED_WILL_BE_REMOVED(CustomElementProcessingStep);
     WTF_MAKE_NONCOPYABLE(CustomElementProcessingStep);
 public:
     CustomElementProcessingStep() { }