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.

components/style/properties/declaration_block.rs

@@ -369,7 +369,7 @@ impl PropertyDeclarationBlock {
                 return false
             }
         }
-        self.declarations.iter().any(|&&(ref decl, _)|
+        self.declarations.iter().any(|&(ref decl, _)|
             decl.id().is_or_is_longhand_of(property) &&
             decl.get_css_wide_keyword().is_some()
         )

components/style/stylist.rs

@@ -854,6 +854,33 @@ impl Stylist {
     {
         self.dependencies.compute_hint(element, snapshot)
     }
+
+    /// Computes styles for a given declaration with parent_style.
+    pub fn compute_for_declarations(&self,
+                                    guards: &StylesheetGuards,
+                                    parent_style: &Arc<ComputedValues>,
+                                    declarations: Arc<Locked<PropertyDeclarationBlock>>)
+                                    -> Arc<ComputedValues> {
+        use font_metrics::get_metrics_provider_for_product;
+
+        let v = vec![
+            ApplicableDeclarationBlock::from_declarations(declarations.clone(),
+                                                          CascadeLevel::StyleAttributeNormal)
+        ];
+        let rule_node =
+            self.rule_tree.insert_ordered_rules(v.into_iter().map(|a| (a.source, a.level)));
+
+        let metrics = get_metrics_provider_for_product();
+        Arc::new(properties::cascade(&self.device,
+                                     &rule_node,
+                                     guards,
+                                     Some(parent_style),
+                                     Some(parent_style),
+                                     None,
+                                     &StdoutErrorReporter,
+                                     &metrics,
+                                     CascadeFlags::empty()))
+    }
 }
 
 impl Drop for Stylist {

ports/geckolib/glue.rs

@@ -2123,3 +2123,27 @@ pub extern "C" fn Servo_StyleSet_GetFontFaceRules(raw_data: RawServoStyleSetBorr
         dest.mSheetType = src.1.into();
     }
 }
+
+#[no_mangle]
+pub extern "C" fn Servo_StyleSet_ResolveForDeclarations(raw_data: RawServoStyleSetBorrowed,
+                                                        parent_style_or_null: ServoComputedValuesBorrowedOrNull,
+                                                        declarations: RawServoDeclarationBlockBorrowed)
+                                                        -> ServoComputedValuesStrong
+{
+    let doc_data = PerDocumentStyleData::from_ffi(raw_data).borrow();
+    let global_style_data = &*GLOBAL_STYLE_DATA;
+    let guard = global_style_data.shared_lock.read();
+    let guards = StylesheetGuards::same(&guard);
+
+    let parent_style = match ComputedValues::arc_from_borrowed(&parent_style_or_null) {
+        Some(parent) => &parent,
+        None => doc_data.default_computed_values(),
+    };
+
+    let declarations = Locked::<PropertyDeclarationBlock>::as_arc(&declarations);
+
+    doc_data.stylist.compute_for_declarations(&guards,
+                                              parent_style,
+                                              declarations.clone()).into_strong()
+}
+