Move content's shell connections to the IO thread

services/shell/public/cpp/interface_provider.h

Index: services/shell/public/cpp/interface_provider.h
diff --git a/services/shell/public/cpp/interface_provider.h b/services/shell/public/cpp/interface_provider.h
index fe6930e3f825d8a262ca4b4ed20cb13795958c95..67ae34fd294c7ec5bfbcd50db36ef9a1402795c1 100644
--- a/services/shell/public/cpp/interface_provider.h
+++ b/services/shell/public/cpp/interface_provider.h
@@ -18,6 +18,8 @@ namespace shell {
 // Connection.
 class InterfaceProvider {
  public:
+  using ForwardCallback = base::Callback<void(const mojo::String&,
+                                              mojo::ScopedMessagePipeHandle)>;
   class TestApi {
    public:
     explicit TestApi(InterfaceProvider* provider) : provider_(provider) {}
@@ -41,8 +43,16 @@ class InterfaceProvider {
   InterfaceProvider();
   ~InterfaceProvider();
 
+  // Binds this InterfaceProvider to an actual mojom::InterfaceProvider pipe.
+  // It is an error to call this on a forwarding InterfaceProvider, i.e. this
+  // call is exclusive to Forward().
   void Bind(mojom::InterfaceProviderPtr interface_provider);
 
+  // Sets this InterfaceProvider to forward all GetInterface() requests to
+  // |callback|. It is an error to call this on a bound InterfaceProvider, i.e.
+  // this call is exclusive to Bind().
+  void Forward(const ForwardCallback& callback);
+
   // Returns a raw pointer to the remote InterfaceProvider.
   mojom::InterfaceProvider* get() { return interface_provider_.get(); }