DevTools: allow embedder handling async remote debugger commands.

content/public/browser/devtools_manager_delegate.cc

Index: content/public/browser/devtools_manager_delegate.cc
diff --git a/content/public/browser/devtools_manager_delegate.cc b/content/public/browser/devtools_manager_delegate.cc
index b4d12ed947a1b8851baba0c75c5accdf20e1586e..40bdfb99a5c2c3857482638a308e5d5b932853ae 100644
--- a/content/public/browser/devtools_manager_delegate.cc
+++ b/content/public/browser/devtools_manager_delegate.cc
@@ -39,6 +39,12 @@ base::DictionaryValue* DevToolsManagerDelegate::HandleCommand(
   return nullptr;
 }
 
+bool DevToolsManagerDelegate::HandleAsyncCommand(DevToolsAgentHost* agent_host,
+                                                 base::DictionaryValue* command,
+                                                 CommandCallback callback) {
+  return false;
+}
+
 std::string DevToolsManagerDelegate::GetDiscoveryPageHTML() {
   return std::string();
 }