Add interfaces for most major Blimp net components.

blimp/engine/browser/blimp_engine_session.cc

Index: blimp/engine/browser/blimp_engine_session.cc
diff --git a/blimp/engine/browser/blimp_engine_session.cc b/blimp/engine/browser/blimp_engine_session.cc
index e4744d9c8d5a3d6631f56c120e531bbbdbb2d89f..7ec7f6f942eee26a1f3cc305dba46955689d697f 100644
--- a/blimp/engine/browser/blimp_engine_session.cc
+++ b/blimp/engine/browser/blimp_engine_session.cc
@@ -17,6 +17,7 @@
 #include "content/public/browser/render_view_host.h"
 #include "content/public/browser/render_widget_host.h"
 #include "content/public/browser/web_contents.h"
+#include "net/base/net_errors.h"
 #include "ui/aura/client/default_capture_client.h"
 #include "ui/aura/env.h"
 #include "ui/aura/window.h"
@@ -96,11 +97,11 @@ void BlimpEngineSession::AttachClientConnection(
   BlimpMessage message;
   message.set_type(BlimpMessage::CONTROL);
   message.mutable_control()->set_type(ControlMessage::CREATE_TAB);
-  OnBlimpMessage(message);
+  ProcessMessage(message, net::CompletionCallback());
   message.mutable_control()->set_type(ControlMessage::LOAD_URL);
   message.mutable_control()->mutable_load_url()->set_url(
       "https://www.google.com/");
-  OnBlimpMessage(message);
+  ProcessMessage(message, net::CompletionCallback());
 }
 
 void BlimpEngineSession::CreateWebContents(const int target_tab_id) {
@@ -124,7 +125,9 @@ void BlimpEngineSession::LoadUrl(const int target_tab_id, const GURL& url) {
   web_contents_->Focus();
 }
 
-net::Error BlimpEngineSession::OnBlimpMessage(const BlimpMessage& message) {
+void BlimpEngineSession::ProcessMessage(
+    const BlimpMessage& message,
+    const net::CompletionCallback& callback) {
   DCHECK(message.type() == BlimpMessage::CONTROL);
 
   switch (message.control().type()) {
@@ -139,7 +142,7 @@ net::Error BlimpEngineSession::OnBlimpMessage(const BlimpMessage& message) {
       NOTIMPLEMENTED();
   }
 
-  return net::OK;
+  callback.Run(net::OK);
 }
 
 void BlimpEngineSession::AddNewContents(content::WebContents* source,