Issue #2736 - Part 2: Pass subject principal to SetAttribute and friends.

In order to tailor certain security checks to the caller that is
attempting to load a particular piece of content, we need to be able to
attach an appropriate triggering principal to the corresponding requests.

Since most HTML content is loaded based on attribute values, this means
capturing the subject principal of the caller who sets those attributes,
which in turn means we need to make it available to `AfterSetAttr` hooks
on all relevant element types.

dom/webidl/AnonymousContent.webidl

@@ -42,7 +42,7 @@ interface AnonymousContent {
    * Set the value of an attribute of an element inside this custom anonymous
    * content.
    */
-  [Throws]
+  [NeedsSubjectPrincipal, Throws]
   void setAttributeForElement(DOMString elementId,
                               DOMString attributeName,
                               DOMString value);