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 {