Bug 1571650: Add an API to change the style of AnonymousContent from devtools. r=smaug

DevTools is using AnonymousContent to highlight the element inspecting.
And now, the z-order of highlighting became to be importance. In case of normal
element, we can change the z-order by z-index or order of DOM. However,
AnonymousContent does not have the such the way to change the z-order from
DevTools side. This patch introduces an API to change the style, and makes the
z-order changeable by the z-index.

Differential Revision: https://phabricator.services.mozilla.com/D41927

--HG--
extra : moz-landing-system : lando

dom/base/AnonymousContent.cpp

@@ -207,5 +207,17 @@ void AnonymousContent::GetTargetIdForEvent(Event& aEvent, DOMString& aResult) {
   aResult.SetNull();
 }
 
+void AnonymousContent::SetStyle(const nsAString& aProperty,
+                                const nsAString& aValue, ErrorResult& aRv) {
+  if (!mContentNode->IsHTMLElement()) {
+    aRv.Throw(NS_ERROR_NOT_AVAILABLE);
+    return;
+  }
+
+  nsGenericHTMLElement* element = nsGenericHTMLElement::FromNode(mContentNode);
+  nsCOMPtr<nsICSSDeclaration> declaration = element->Style();
+  declaration->SetProperty(aProperty, aValue, EmptyString());
+}
+
 }  // namespace dom
 }  // namespace mozilla

dom/base/AnonymousContent.h

@@ -71,6 +71,9 @@ class AnonymousContent final {
 
   void GetTargetIdForEvent(Event& aEvent, DOMString& aResult);
 
+  void SetStyle(const nsAString& aProperty, const nsAString& aValue,
+                ErrorResult& aRv);
+
  private:
   ~AnonymousContent();
   RefPtr<Element> mContentNode;

dom/webidl/AnonymousContent.webidl

@@ -91,4 +91,10 @@ interface AnonymousContent {
    * attribute value of the target.
    */
   DOMString? getTargetIdForEvent(Event event);
+
+  /**
+   * Set given style to this AnonymousContent.
+   */
+  [Throws]
+  void setStyle(DOMString property, DOMString value);
 };