Change userid from a uint32_t to a string guid

mojo/shell/public/cpp/application_test_base.h

Index: mojo/shell/public/cpp/application_test_base.h
diff --git a/mojo/shell/public/cpp/application_test_base.h b/mojo/shell/public/cpp/application_test_base.h
index 91160df0fbb65944aab5f29e12d62ebcbfa57ef2..958c7795f270d67a55bc46de935f24816f4fbde0 100644
--- a/mojo/shell/public/cpp/application_test_base.h
+++ b/mojo/shell/public/cpp/application_test_base.h
@@ -34,9 +34,9 @@ class TestHelper {
   ~TestHelper();
 
   Connector* connector() { return shell_connection_->connector(); }
-  std::string test_name() { return name_; }
+  const std::string& test_name() { return name_; }
+  const std::string& test_userid() { return userid_; }
   uint32_t test_instance_id() { return instance_id_; }
-  uint32_t test_userid() { return userid_; }
 
  private:
   // The application delegate used if GetShellClient is not overridden.
@@ -46,8 +46,8 @@ class TestHelper {
   scoped_ptr<ShellConnection> shell_connection_;
 
   std::string name_;
+  std::string userid_;
   uint32_t instance_id_;
-  uint32_t userid_;
 
   DISALLOW_COPY_AND_ASSIGN(TestHelper);
 };
@@ -62,17 +62,16 @@ class ApplicationTestBase : public testing::Test {
   Connector* connector() {
     return test_helper_ ? test_helper_->connector() : nullptr;
   }
-  std::string test_name() const {
-    return test_helper_ ? test_helper_->test_name() : std::string();
+  const std::string& test_name() const {
+    return test_helper_ ? test_helper_->test_name() : empty_;
+  }
+  const std::string& test_userid() const {
+    return test_helper_ ? test_helper_->test_userid() : inherit_user_id_;
   }
   uint32_t test_instance_id() const {
     return test_helper_ ? test_helper_->test_instance_id() :
         shell::mojom::Connector::kInvalidApplicationID;
   }
-  uint32_t test_userid() const {
-    return test_helper_ ? test_helper_->test_userid() :
-        shell::mojom::Connector::kUserInherit;
-  }
 
   // Get the ShellClient for the application to be tested.
   virtual ShellClient* GetShellClient();
@@ -88,6 +87,8 @@ class ApplicationTestBase : public testing::Test {
 
  private:
   scoped_ptr<TestHelper> test_helper_;
+  std::string empty_;
+  std::string inherit_user_id_ = shell::mojom::kInheritUserID;
 
   DISALLOW_COPY_AND_ASSIGN(ApplicationTestBase);
 };