Bug 1254857. Switch the AutoEntryScript in xpc::EvalInSandbox to take ownership of error reporting. r=bholley

In practice we always propagate the exception out anyway, so this change is a no-op.

js/xpconnect/src/Sandbox.cpp

@@ -1703,6 +1703,7 @@ xpc::EvalInSandbox(JSContext* cx, HandleObject sandboxArg, const nsAString& sour
         // We're about to evaluate script, so make an AutoEntryScript.
         // This is clearly Gecko-specific and not in any spec.
         mozilla::dom::AutoEntryScript aes(priv, "XPConnect sandbox evaluation");
+        aes.TakeOwnershipOfErrorReporting();
         JSContext* sandcx = aes.cx();
         AutoSaveContextOptions savedOptions(sandcx);
         JS::ContextOptionsRef(sandcx).setDontReportUncaught(true);
@@ -1716,9 +1717,8 @@ xpc::EvalInSandbox(JSContext* cx, HandleObject sandboxArg, const nsAString& sour
                           PromiseFlatString(source).get(), source.Length(), &v);
 
         // If the sandbox threw an exception, grab it off the context.
-        if (JS_GetPendingException(sandcx, &exn)) {
-            MOZ_ASSERT(!ok);
-            JS_ClearPendingException(sandcx);
+        if (aes.HasException()) {
+            aes.StealException(&exn);
         }
     }