Bug 865729 - Make nsIXPConnect inherit nsIThreadJSContextStack. r=gabor

Right now, the concrete nsXPConnect implements nsIXPConnect and
nsIThreadJSContextStack separately. I want to migrate the API from the latter
interface to the former, but I can't right now because it means we'd end up
with a duplicated method (getSafeJSContext). Since there's only one concrete
class that implements nsXPConnect, let's just use interface inheritance, which
simplifies the migration.

js/xpconnect/idl/nsIXPConnect.idl

@@ -17,6 +17,7 @@
 #include "nsIVariant.idl"
 #include "nsIObjectOutputStream.idl"
 #include "nsIObjectInputStream.idl"
+#include "nsIJSContextStack.idl"
 
 %{ C++
 #include "jspubtd.h"
@@ -290,8 +291,8 @@ interface nsIXPCFunctionThisTranslator : nsISupports
     { 0xbd, 0xd6, 0x0, 0x0, 0x64, 0x65, 0x73, 0x74 } }
 %}
 
-[uuid(7e7a9a90-4607-4e3a-8ef1-b0f4b8de1c27)]
-interface nsIXPConnect : nsISupports
+[uuid(b3012603-aa25-4192-b331-c16272499957)]
+interface nsIXPConnect : nsIThreadJSContextStack
 {
 %{ C++
   NS_DEFINE_STATIC_CID_ACCESSOR(NS_XPCONNECT_CID)

js/xpconnect/src/xpcprivate.h

@@ -464,7 +464,6 @@ class nsXPConnect : public nsIXPConnect,
                     public nsSupportsWeakReference,
                     public nsCycleCollectionJSRuntime,
                     public nsIJSRuntimeService,
-                    public nsIThreadJSContextStack,
                     public nsIJSEngineTelemetryStats
 {
 public: