Bug 884245 - Simplify nsContentUtils::GetDocumentFromCaller. r=bz

The first three lines of this function are an obvious bug. But the behavior in
general is also worrisome, because it's using antiquated XPConnect machinery
that will be less and less correct as we move to the new DOM bindings. Let's
just infer our global from the active compartment.

content/base/src/nsContentUtils.cpp

@@ -1671,15 +1671,10 @@ nsContentUtils::GetWindowFromCaller()
 nsIDocument*
 nsContentUtils::GetDocumentFromCaller()
 {
-  JSContext *cx = nullptr;
-  JS::Rooted<JSObject*> obj(cx);
-  sXPConnect->GetCaller(&cx, obj.address());
-  NS_ASSERTION(cx && obj, "Caller ensures something is running");
-
-  JSAutoCompartment ac(cx, obj);
+  AutoJSContext cx;
 
   nsCOMPtr<nsPIDOMWindow> win =
-    do_QueryInterface(nsJSUtils::GetStaticScriptGlobal(obj));
+    do_QueryInterface(nsJSUtils::GetStaticScriptGlobal(JS_GetGlobalForScopeChain(cx)));
   if (!win) {
     return nullptr;
   }

js/xpconnect/idl/nsIXPConnect.idl

@@ -291,7 +291,7 @@ interface nsIXPCFunctionThisTranslator : nsISupports
     { 0xbd, 0xd6, 0x0, 0x0, 0x64, 0x65, 0x73, 0x74 } }
 %}
 
-[uuid(4498aa26-62df-4a4f-8a45-7ddfd7f84834)]
+[uuid(dee6f80b-7ec5-4690-8622-a48af6a1ce52)]
 interface nsIXPConnect : nsISupports
 {
 %{ C++
@@ -609,12 +609,6 @@ interface nsIXPConnect : nsISupports
     nsIXPConnectJSObjectHolder holdObject(in JSContextPtr aJSContext,
                                           in JSObjectPtr aObject);
 
-    /**
-     * Return the caller object of the current call from JS.
-     */
-    [noscript,notxpcom] void getCaller(out JSContextPtr aJSContext,
-                                       out JSObjectPtr aObject);
-
     /**
      * When we place the browser in JS debug mode, there can't be any
      * JS on the stack. This is because we currently activate debugMode 

js/xpconnect/src/nsXPConnect.cpp

@@ -1346,16 +1346,6 @@ nsXPConnect::HoldObject(JSContext *aJSContext, JSObject *aObjectArg,
     return NS_OK;
 }
 
-NS_IMETHODIMP_(void)
-nsXPConnect::GetCaller(JSContext **aJSContext, JSObject **aObj)
-{
-    XPCCallContext *ccx = XPCJSRuntime::Get()->GetCallContext();
-    *aJSContext = ccx->GetJSContext();
-
-    // Set to the caller in XPC_WN_Helper_{Call,Construct}
-    *aObj = ccx->GetFlattenedJSObject();
-}
-
 namespace xpc {
 
 bool