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

blimp/engine/app/blimp_content_main_delegate.h

Index: blimp/engine/app/blimp_content_main_delegate.h
diff --git a/blimp/engine/app/blimp_content_main_delegate.h b/blimp/engine/app/blimp_content_main_delegate.h
index bc7876fc2f78d5f1eb007c7f758dd1a61a532735..3de47c085a5e4c1cffd57abb2d24d4c910052934 100644
--- a/blimp/engine/app/blimp_content_main_delegate.h
+++ b/blimp/engine/app/blimp_content_main_delegate.h
@@ -22,6 +22,8 @@ class BlimpContentMainDelegate : public content::ContentMainDelegate {
   BlimpContentMainDelegate();
   ~BlimpContentMainDelegate() override;
 
+  BlimpContentBrowserClient* browser_client() { return browser_client_.get(); }
+
   // content::ContentMainDelegate implementation.
   bool BasicStartupComplete(int* exit_code) override;
   void PreSandboxStartup() override;