Bug 1294299 part 3 - Make it possible to create empty ServoDeclarationBlock. r=heycam

MozReview-Commit-ID: FSdSXDFoxM

--HG--
extra : source : 47bdaffadab2dca8be8b6e49c7decf8ef02ea1e5

layout/style/ServoBindingList.h

@@ -62,6 +62,8 @@ SERVO_BINDING_FUNC(Servo_RestyleWithAddedDeclaration,
 // Style attribute
 SERVO_BINDING_FUNC(Servo_ParseStyleAttribute, RawServoDeclarationBlockStrong,
                    const nsACString* data)
+SERVO_BINDING_FUNC(Servo_DeclarationBlock_CreateEmpty,
+                   RawServoDeclarationBlockStrong)
 SERVO_BINDING_FUNC(Servo_DeclarationBlock_AddRef, void,
                    RawServoDeclarationBlockBorrowed declarations)
 SERVO_BINDING_FUNC(Servo_DeclarationBlock_Release, void,

layout/style/ServoDeclarationBlock.h

@@ -14,6 +14,9 @@ namespace mozilla {
 class ServoDeclarationBlock final : public DeclarationBlock
 {
 public:
+  ServoDeclarationBlock()
+    : ServoDeclarationBlock(Servo_DeclarationBlock_CreateEmpty().Consume()) {}
+
   NS_INLINE_DECL_REFCOUNTING(ServoDeclarationBlock)
 
   static already_AddRefed<ServoDeclarationBlock>

layout/style/nsDOMCSSAttrDeclaration.cpp

@@ -128,8 +128,13 @@ nsDOMCSSAttributeDeclaration::GetCSSDeclaration(Operation aOperation)
   }
 
   // cannot fail
-  RefPtr<css::Declaration> decl = new css::Declaration();
-  decl->InitializeEmpty();
+  RefPtr<DeclarationBlock> decl;
+  if (mElement->IsStyledByServo()) {
+    decl = new ServoDeclarationBlock();
+  } else {
+    decl = new css::Declaration();
+    decl->AsGecko()->InitializeEmpty();
+  }
 
   // this *can* fail (inside SetAttrAndNotify, at least).
   nsresult rv;