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

blimp/engine/app/switches.h

Index: blimp/engine/app/switches.h
diff --git a/blimp/engine/app/switches.h b/blimp/engine/app/switches.h
index 6e25d738e1ee6a102a8127fae73b1a2085d36cb1..ba6e038c3e26e277e9f539e50d697a953c1881b2 100644
--- a/blimp/engine/app/switches.h
+++ b/blimp/engine/app/switches.h
@@ -11,6 +11,10 @@ namespace engine {
 // Path to the client token/shared secret between the engine and the client.
 extern const char kClientTokenPath[];
 
+// The client token/shared secret between the engine and the client.
+// BrowserTest uses it to get the token.
+extern const char kClientToken[];
+
 // The port to listen to for incoming TCP connections.
 extern const char kEnginePort[];