heap: Make all flags used for TraceAfterDispatch const

TraceAfterDispatch is used avoid adding vtables to types. Instead a
flag in the object is used to choose the right Trace method. This CL
changes all the non-const flags used for dispatching to const.

Change-Id: I29b1817b56808360c27b31dd41b98c5ae5ece1a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078533Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745062}

third_party/blink/renderer/core/css/css_property_value_set.h

@@ -159,7 +159,7 @@ class CORE_EXPORT CSSPropertyValueSet
   }
 
   unsigned css_parser_mode_ : 3;
-  mutable unsigned is_mutable_ : 1;
+  const unsigned is_mutable_ : 1;
   unsigned array_size_ : 28;
 
   friend class PropertySetCSSStyleDeclaration;

third_party/blink/renderer/core/css/css_value.h

@@ -294,7 +294,7 @@ class CORE_EXPORT CSSValue : public GarbageCollected<CSSValue> {
   unsigned allows_negative_percentage_reference_ : 1;
 
  private:
-  unsigned class_type_ : kClassTypeBits;  // ClassType
+  const unsigned class_type_ : kClassTypeBits;  // ClassType
 };
 
 template <typename CSSValueType, wtf_size_t inlineCapacity>

third_party/blink/renderer/core/css/style_rule.h

@@ -91,7 +91,7 @@ class CORE_EXPORT StyleRuleBase : public GarbageCollected<StyleRuleBase> {
   CSSRule* CreateCSSOMWrapper(CSSStyleSheet* parent_sheet,
                               CSSRule* parent_rule) const;
 
-  unsigned type_ : 5;
+  const unsigned type_ : 5;
 };
 
 // A single rule from a stylesheet. Contains a selector list (one or more

third_party/blink/renderer/core/dom/element_data.cc

@@ -111,10 +111,13 @@ bool ElementData::IsEquivalent(const ElementData* other) const {
 }
 
 void ElementData::Trace(Visitor* visitor) {
-  if (auto* unique_element_data = DynamicTo<UniqueElementData>(this))
-    unique_element_data->TraceAfterDispatch(visitor);
-  else
+  if (is_unique_) {
+    DCHECK(DynamicTo<UniqueElementData>(this));
+    To<UniqueElementData>(this)->TraceAfterDispatch(visitor);
+  } else {
+    DCHECK(DynamicTo<ShareableElementData>(this));
     To<ShareableElementData>(this)->TraceAfterDispatch(visitor);
+  }
 }
 
 void ElementData::TraceAfterDispatch(blink::Visitor* visitor) {

third_party/blink/renderer/core/dom/element_data.h

@@ -90,7 +90,7 @@ class ElementData : public GarbageCollected<ElementData> {
 
   // Keep the type in a bitfield instead of using virtual destructors to avoid
   // adding a vtable.
-  unsigned is_unique_ : 1;
+  const unsigned is_unique_ : 1;
   unsigned array_size_ : 28;
   mutable unsigned presentation_attribute_style_is_dirty_ : 1;
   mutable unsigned style_attribute_is_dirty_ : 1;