Add interfaces for most major Blimp net components.

blimp/engine/browser/blimp_engine_session.h

Index: blimp/engine/browser/blimp_engine_session.h
diff --git a/blimp/engine/browser/blimp_engine_session.h b/blimp/engine/browser/blimp_engine_session.h
index 7a36b74ac294a87499a8f2707b1507fef875d5ce..d8b1574d69e92cd1dcb686bf6ca89217ea5bf73f 100644
--- a/blimp/engine/browser/blimp_engine_session.h
+++ b/blimp/engine/browser/blimp_engine_session.h
@@ -7,7 +7,7 @@
 
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
-#include "blimp/net/blimp_message_receiver.h"
+#include "blimp/net/blimp_message_processor.h"
 #include "content/public/browser/web_contents_delegate.h"
 #include "ui/gfx/geometry/size.h"
 
@@ -42,7 +42,7 @@ class BlimpScreen;
 class BlimpUiContextFactory;
 class BlimpWindowTreeHost;
 
-class BlimpEngineSession : public BlimpMessageReceiver,
+class BlimpEngineSession : public BlimpMessageProcessor,
                            public content::WebContentsDelegate {
  public:
   explicit BlimpEngineSession(scoped_ptr<BlimpBrowserContext> browser_context);
@@ -61,8 +61,9 @@ class BlimpEngineSession : public BlimpMessageReceiver,
   // Navigates the target tab to the |url|.
   void LoadUrl(const int target_tab_id, const GURL& url);
 
-  // BlimpMessageReceiver implementation.
-  net::Error OnBlimpMessage(const BlimpMessage& message) override;
+  // BlimpMessageProcessor implementation.
+  void ProcessMessage(const BlimpMessage& message,
+                      const net::CompletionCallback& callback) override;
 
   // content::WebContentsDelegate implementation.
   content::WebContents* OpenURLFromTab(