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

blimp/engine/app/blimp_content_browser_client.h

Index: blimp/engine/app/blimp_content_browser_client.h
diff --git a/blimp/engine/app/blimp_content_browser_client.h b/blimp/engine/app/blimp_content_browser_client.h
index 87d174ce8360738850396c478aca492e7abb4556..ab5e89578e5d1e634a3e13afc166cfe3a05ab1d5 100644
--- a/blimp/engine/app/blimp_content_browser_client.h
+++ b/blimp/engine/app/blimp_content_browser_client.h
@@ -19,6 +19,10 @@ class BlimpContentBrowserClient : public content::ContentBrowserClient {
   BlimpContentBrowserClient();
   ~BlimpContentBrowserClient() override;
 
+  BlimpBrowserMainParts* blimp_browser_main_parts() {
+    return blimp_browser_main_parts_;
+  }
+
   // content::ContentBrowserClient implementation.
   content::BrowserMainParts* CreateBrowserMainParts(
       const content::MainFunctionParams& parameters) override;