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

chrome/browser/chrome_content_browser_client.cc

Index: chrome/browser/chrome_content_browser_client.cc
diff --git a/chrome/browser/chrome_content_browser_client.cc b/chrome/browser/chrome_content_browser_client.cc
index f7c3c618d8c8ecbcf10cbc566c92548eba06dde1..86aff4941fc2b32f46533ef4a4f04259c8038301 100644
--- a/chrome/browser/chrome_content_browser_client.cc
+++ b/chrome/browser/chrome_content_browser_client.cc
@@ -818,9 +818,10 @@ std::string ChromeContentBrowserClient::GetStoragePartitionIdForSite(
   // SiteInstance URL - "chrome-guest://app_id/persist?partition".
   if (site.SchemeIs(content::kGuestScheme)) {
     partition_id = site.spec();
-  } else if (site.GetOrigin().spec() == kChromeUIChromeSigninURL) {
-    // Chrome signin page has an embedded iframe of extension and web content,
-    // thus it must be isolated from other webUI pages.
+  } else if (!switches::IsEnableWebviewBasedSignin() &&
+             site.GetOrigin().spec() == kChromeUIChromeSigninURL) {
+    // The non-webview Chrome signin page has an embedded iframe of extension
+    // and web content, thus it must be isolated from other webUI pages.
     partition_id = site.GetOrigin().spec();
   }
 
@@ -882,7 +883,8 @@ void ChromeContentBrowserClient::GetStoragePartitionConfigForSite(
   }
 #endif
 
-  if (!success && (site.GetOrigin().spec() == kChromeUIChromeSigninURL)) {
+  if (!success && (!switches::IsEnableWebviewBasedSignin() &&
+                   site.GetOrigin().spec() == kChromeUIChromeSigninURL)) {
     // Chrome signin page has an embedded iframe of extension and web content,

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

Please update this comment as well.

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

Done.

     // thus it must be isolated from other webUI pages.
     *partition_domain = chrome::kChromeUIChromeSigninHost;