DevTools: return discovered targets asynchronously.

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 5b366b548cff3e2158c7de9b31a86fdda4a95303..d3a9608fb09c090136833945821099de88a124c2 100644
--- a/content/public/browser/devtools_manager_delegate.cc
+++ b/content/public/browser/devtools_manager_delegate.cc
@@ -27,6 +27,11 @@ std::string DevToolsManagerDelegate::GetTargetDescription(
   return std::string();
 }
 
+bool DevToolsManagerDelegate::DiscoverTargets(
+    const DevToolsAgentHost::DiscoveryCallback& callback) {
+  return false;
+}
+
 scoped_refptr<DevToolsAgentHost> DevToolsManagerDelegate::CreateNewTarget(
     const GURL& url) {
   return nullptr;