Bug 1324700 - Add an FFI which returns computed values for a given declaration block with/without parent_style. r=heycam

The function uses document's default computed values if the parent style
is not specified.

MozReview-Commit-ID: ICd3phAi0C6

--HG--
extra : rebase_source : 343dee682096b75cd7f905db7207823f7e3624b5

layout/style/ServoBindingList.h

@@ -70,6 +70,11 @@ SERVO_BINDING_FUNC(Servo_StyleSet_FillKeyframesForName, bool,
 SERVO_BINDING_FUNC(Servo_StyleSet_GetFontFaceRules, void,
                    RawServoStyleSetBorrowed set,
                    RawGeckoFontFaceRuleListBorrowedMut list)
+SERVO_BINDING_FUNC(Servo_StyleSet_ResolveForDeclarations,
+                   ServoComputedValuesStrong,
+                   RawServoStyleSetBorrowed set,
+                   ServoComputedValuesBorrowedOrNull parent_style,
+                   RawServoDeclarationBlockBorrowed declarations)
 
 // CSSRuleList
 SERVO_BINDING_FUNC(Servo_CssRules_ListTypes, void,

layout/style/ServoStyleSet.cpp

@@ -924,4 +924,14 @@ ServoStyleSet::AppendFontFaceRules(nsTArray<nsFontFaceRuleContainer>& aArray)
   return true;
 }
 
+already_AddRefed<ServoComputedValues>
+ServoStyleSet::ResolveForDeclarations(
+  ServoComputedValuesBorrowedOrNull aParentOrNull,
+  RawServoDeclarationBlockBorrowed aDeclarations)
+{
+  return Servo_StyleSet_ResolveForDeclarations(mRawSet.get(),
+                                               aParentOrNull,
+                                               aDeclarations).Consume();
+}
+
 bool ServoStyleSet::sInServoTraversal = false;

layout/style/ServoStyleSet.h

@@ -297,6 +297,15 @@ public:
   already_AddRefed<ServoComputedValues>
   GetBaseComputedValuesForElement(dom::Element* aElement, nsIAtom* aPseudoTag);
 
+  /**
+   * Resolve style for a given declaration block with/without the parent style.
+   * If the parent style is not specified, the document default computed values
+   * is used.
+   */
+  already_AddRefed<ServoComputedValues>
+  ResolveForDeclarations(ServoComputedValuesBorrowedOrNull aParentOrNull,
+                         RawServoDeclarationBlockBorrowed aDeclarations);
+
 private:
   already_AddRefed<nsStyleContext> GetContext(already_AddRefed<ServoComputedValues>,
                                               nsStyleContext* aParentContext,