Deliver errors from devtools API calls to callbacks

headless/lib/browser/devtools_api/domain_cc.template

Index: headless/lib/browser/devtools_api/domain_cc.template
diff --git a/headless/lib/browser/devtools_api/domain_cc.template b/headless/lib/browser/devtools_api/domain_cc.template
index 88f28b1f56a2d15ba60301d3e761a2dfe9050670..3345a4ad565a73e0291f7c92ed61cab543d04bc1 100644
--- a/headless/lib/browser/devtools_api/domain_cc.template
+++ b/headless/lib/browser/devtools_api/domain_cc.template
@@ -102,6 +102,11 @@ void {{class_name}}::{{method_name}}(std::unique_ptr<{{method_name}}Params> para
 void Domain::Handle{{method_name}}Response(base::Callback<void(std::unique_ptr<{{method_name}}Result>)> callback, const base::Value& response) {
   if (callback.is_null())
     return;
+  // This is an error response.
+  if (response.IsType(base::Value::TYPE_NULL)) {
+    callback.Run(nullptr);
+    return;
+  }
   ErrorReporter errors;
   std::unique_ptr<{{method_name}}Result> result = {{method_name}}Result::Parse(response, &errors);
   DCHECK(!errors.HasErrors());