[idlharness.js] Improve error msg for invalid objects.

Note that this is not a functional change; it just helps authors of idlharness tests identify what's gone wrong if they have incorrect add_objects params.
testharness.js drops the stack for thrown errors and prints a (useless) new stack.

Change-Id: I6dd90c461f0ab58e57978a28e3e5e9eb366a4827
Reviewed-on: https://chromium-review.googlesource.com/1104311
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568803}

third_party/WebKit/LayoutTests/external/wpt/resources/idlharness.js

@@ -821,8 +821,15 @@ IdlArray.prototype.test = function()
         this.members[name].test();
         if (name in this.objects)
         {
-            this.objects[name].forEach(function(str)
+            const objects = this.objects[name];
+            if (!objects || !Array.isArray(objects)) {
+                throw new IdlHarnessError(`Invalid or empty objects for member ${name}`);
+            }
+            objects.forEach(function(str)
             {
+                if (!this.members[name] || !(this.members[name] instanceof IdlInterface)) {
+                    throw new IdlHarnessError(`Invalid object member name ${name}`);
+                }
                 this.members[name].test_object(str);
             }.bind(this));
         }