Bug 1479712 part 1 - Require a plain function in xpcshell setInterruptCallback. r=bholley

The head_watchdog.js test always calls this with a function argument. This lets us use JSAutoRealm instead of JSAutoRealmAllowCCW.

js/xpconnect/src/XPCShellImpl.cpp

@@ -542,7 +542,9 @@ XPCShellInterruptCallback(JSContext* cx)
     if (callback.isUndefined())
         return true;
 
-    JSAutoRealmAllowCCW ar(cx, &callback.toObject());
+    MOZ_ASSERT(js::IsFunctionObject(&callback.toObject()));
+
+    JSAutoRealm ar(cx, &callback.toObject());
     RootedValue rv(cx);
     if (!JS_CallFunctionValue(cx, nullptr, callback, JS::HandleValueArray::empty(), &rv) ||
         !rv.isBoolean())
@@ -573,9 +575,9 @@ SetInterruptCallback(JSContext* cx, unsigned argc, Value* vp)
         return true;
     }
 
-    // Otherwise, we should have a callable object.
-    if (!args[0].isObject() || !JS::IsCallable(&args[0].toObject())) {
-        JS_ReportErrorASCII(cx, "Argument must be callable");
+    // Otherwise, we should have a function object.
+    if (!args[0].isObject() || !js::IsFunctionObject(&args[0].toObject())) {
+        JS_ReportErrorASCII(cx, "Argument must be a function");
         return false;
     }