Webview-based signin shouldn't create a partition for the embedder.

components/signin/core/common/profile_management_switches.cc

Index: components/signin/core/common/profile_management_switches.cc
diff --git a/components/signin/core/common/profile_management_switches.cc b/components/signin/core/common/profile_management_switches.cc
index 88c65abb0a555a59eb1397bc36e493f8fe002f9c..6e9702eafaf0629ab898eed55f1cbee526259174 100644
--- a/components/signin/core/common/profile_management_switches.cc
+++ b/components/signin/core/common/profile_management_switches.cc
@@ -164,6 +164,10 @@ bool IsNewProfileManagementPreviewEnabled() {
   return false;
 }
 
+void EnableIframeBasedSigninForTesting(base::CommandLine* command_line) {
+  command_line->AppendSwitch(switches::kEnableIframeBasedSignin);

[Charlie Reis, 2015/03/11 20:23:10]

Is there a reason you need this method rather than just calling
command_line->AppendSwitch directly in your test class?  (That's what we do in
SitePerProcessBrowserTest, for example.)

If you do need this, then you should probably have a similar DCHECK as the
methods below for the IsEnableWebviewBasedSignin case.

[wjmaclean, 2015/03/23 15:31:52]

I can do it directly in the test ... I was just following an existing pattern
:-)

+}
+
 void EnableNewAvatarMenuForTesting(base::CommandLine* command_line) {
   command_line->AppendSwitch(switches::kEnableNewAvatarMenu);
   DCHECK(!command_line->HasSwitch(switches::kDisableNewAvatarMenu));