[Blimp] Adds blimp engine browser test framework and LoadUrl test.

blimp/engine/session/blimp_engine_session.h

Index: blimp/engine/session/blimp_engine_session.h
diff --git a/blimp/engine/session/blimp_engine_session.h b/blimp/engine/session/blimp_engine_session.h
index a78bb664c4b7a560cf1ca9a07c75f8b76be33b11..03d3e6bf973bc0cf4e63b809655d2b2adb025537 100644
--- a/blimp/engine/session/blimp_engine_session.h
+++ b/blimp/engine/session/blimp_engine_session.h
@@ -67,6 +67,8 @@ class BlimpScreen;
 class BlimpWindowTreeHost;
 class EngineNetworkComponents;
 
+using GetPortCallback = base::Callback<void(uint16_t)>;
+
 class BlimpEngineSession
     : public BlimpMessageProcessor,
       public content::WebContentsDelegate,
@@ -87,6 +89,9 @@ class BlimpEngineSession
 
   BlimpBrowserContext* browser_context() { return browser_context_.get(); }
 
+  // Gets Engine's listening port. Invokes callback with the allocated port.
+  void GetEnginePortForTesting(const GetPortCallback& callback);
+
   // BlimpMessageProcessor implementation.
   // This object handles incoming TAB_CONTROL and NAVIGATION messages directly.
   void ProcessMessage(std::unique_ptr<BlimpMessage> message,