Bug 879475 - Part 005. Implement ContentBridge r=jlebar

Based on original patch by David Zbarsky <dzbarsky@gmail.com>

js/ipc/PJavaScript.ipdl

@@ -6,6 +6,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 include protocol PContent;
+include protocol PContentBridge;
 include DOMTypes;
 include JavaScriptTypes;
 
@@ -16,7 +17,7 @@ namespace jsipc {
 
 intr protocol PJavaScript
 {
-    manager PContent;
+    manager PContent or PContentBridge;
 
 both:
     // Sent when a CPOW has been finalized and table entries can be freed up.