Remove support for Root identity singletons and rely on the all_users bit to be set. + test

mojo/shell/shell.h

Index: mojo/shell/shell.h
diff --git a/mojo/shell/shell.h b/mojo/shell/shell.h
index 7128a24ff5a8c9b39979eb60522ef25906446a33..8b9f975275c62fb52d69f41cf1d8d0acc802b948 100644
--- a/mojo/shell/shell.h
+++ b/mojo/shell/shell.h
@@ -114,11 +114,9 @@ class Shell : public ShellClient {
   // |client| to control it.
   void Connect(scoped_ptr<ConnectParams> params, mojom::ShellClientPtr client);
 
-  // Returns a running instance matching |identity|.
+  // Returns a running instance matching |identity|. This might be an instance
+  // running as a different user if one is available that services all users.
   Instance* GetExistingInstance(const Identity& identity) const;
-  // Like GetExistingInstance, but if no instance for |identity.user_id()| is
-  // found, looks for kRootUserID too.
-  Instance* GetExistingOrRootInstance(const Identity& identity) const;
 
   void NotifyPIDAvailable(uint32_t id, base::ProcessId pid);