[Extensions Bindings] Accept null callbacks while ignoring schema

extensions/renderer/api_signature.cc

Index: extensions/renderer/api_signature.cc
diff --git a/extensions/renderer/api_signature.cc b/extensions/renderer/api_signature.cc
index e15a53f80392707341e8d3c536a8b3a6658bf49f..f4d42c875ab581af8066204c482b3b3984aff644 100644
--- a/extensions/renderer/api_signature.cc
+++ b/extensions/renderer/api_signature.cc
@@ -317,10 +317,11 @@ bool APISignature::ConvertArgumentsIgnoringSchema(
   // 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;
+    DCHECK(value->IsFunction() || value->IsUndefined() || value->IsNull());

[jbroman, 2017/06/05 18:16:32]

This DCHECK implies this value can only come from Chrome code, right? (it's not
provided by the extension author?)

[Devlin, 2017/06/09 20:26:31]

More-or-less. :/  In theory, we should only get here after the extension
bindings have validated the signature, so we should know that it's one of these.
 In practice, there's flaws with this that are described above in the TODO. 
I've added a brief comment here explaining that.

+    if (value->IsFunction())
+      callback = value.As<v8::Function>();
   }
 
   auto json = base::MakeUnique<base::ListValue>();