[Extensions Bindings] Accept null callbacks while ignoring schema

extensions/renderer/api_signature.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "extensions/renderer/api_signature.h"
 
 #include <algorithm>
 
 #include "base/memory/ptr_util.h"
 #include "base/strings/string_util.h"
@@ skipping 299 matching lines @@
   // TODO(devlin): This is what the current bindings do, but it's quite terribly
   // incorrect. We only hit this flow when an API method has a hook to update
   // the arguments post-validation, and in some cases, the arguments returned by
   // that hook do *not* match the signature of the API method (e.g.
   // fileSystem.getDisplayPath); see also note in api_bindings.cc for why this
   // is bad. But then here, we *rely* on the signature to determine whether or
   // not the last parameter is a callback, even though the hooks may not return
   // the arguments in the signature. This is very broken.
   if (HasCallback(signature_)) {
     CHECK(!arguments.empty());
-    if (arguments[size - 1]->IsFunction()) {
-      callback = arguments[size - 1].As<v8::Function>();
-      --size;
-    }
+    v8::Local<v8::Value> value = arguments.back();
+    --size;
+    // Bindings should ensure that the value here is appropriate, but see the
+    // comment above for limitations.
+    DCHECK(value->IsFunction() || value->IsUndefined() || value->IsNull());
+    if (value->IsFunction())
+      callback = value.As<v8::Function>();
   }
 
   auto json = base::MakeUnique<base::ListValue>();
   std::unique_ptr<content::V8ValueConverter> converter(
       content::V8ValueConverter::create());
   converter->SetFunctionAllowed(true);
   for (size_t i = 0; i < size; ++i) {
     std::unique_ptr<base::Value> converted =
         converter->FromV8Value(arguments[i], context);
     if (!converted)
@@ skipping 17 matching lines @@
     pieces.push_back(
         base::StringPrintf("%s%s %s", spec->optional() ? kOptionalPrefix : "",
                            spec->GetTypeName().c_str(), spec->name().c_str()));
   }
   expected_signature_ = base::JoinString(pieces, ", ");
 
   return expected_signature_;
 }
 
 }  // namespace extensions