[Extensions Bindings] Add a bridge to use current custom bindings

extensions/renderer/api_bindings_system.cc

Index: extensions/renderer/api_bindings_system.cc
diff --git a/extensions/renderer/api_bindings_system.cc b/extensions/renderer/api_bindings_system.cc
index 8ab880bb4b3739cba8f9176e179ba3e7349e9d63..5db72a0235820aedf249eff43f77c974d0ec20b1 100644
--- a/extensions/renderer/api_bindings_system.cc
+++ b/extensions/renderer/api_bindings_system.cc
@@ -29,10 +29,13 @@ v8::Local<v8::Object> APIBindingsSystem::CreateAPIInstance(
     const std::string& api_name,
     v8::Local<v8::Context> context,
     v8::Isolate* isolate,
-    const APIBinding::AvailabilityCallback& is_available) {
+    const APIBinding::AvailabilityCallback& is_available,
+    v8::Local<v8::Object>* hooks_interface_out) {
   std::unique_ptr<APIBinding>& binding = api_bindings_[api_name];
   if (!binding)
     binding = CreateNewAPIBinding(api_name);
+  if (hooks_interface_out)
+    *hooks_interface_out = binding->GetJSHookInterface(context);
   return binding->CreateInstance(
       context, isolate, &event_handler_, is_available);
 }
@@ -49,12 +52,17 @@ std::unique_ptr<APIBinding> APIBindingsSystem::CreateNewAPIBinding(
   const base::ListValue* event_definitions = nullptr;
   api_schema.GetList("events", &event_definitions);
 
-  // Find the hooks for the API, if any exist.
+  // Find the hooks for the API. If none exist, an empty set will be created so
+  // we can use JS custom bindings.
+  // TODO(devlin): Once all legacy custom bindings are converted, we don't have
+  // to unconditionally pass in binding hooks.
   std::unique_ptr<APIBindingHooks> hooks;
   auto iter = binding_hooks_.find(api_name);
   if (iter != binding_hooks_.end()) {
     hooks = std::move(iter->second);
     binding_hooks_.erase(iter);
+  } else {
+    hooks = base::MakeUnique<APIBindingHooks>(call_js_);
   }
 
   return base::MakeUnique<APIBinding>(