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

chrome/browser/chrome_content_browser_client.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chrome_content_browser_client.h"
 
 #include <set>
 #include <utility>
 #include <vector>
 
@@ skipping 800 matching lines @@
 
 std::string ChromeContentBrowserClient::GetStoragePartitionIdForSite(
     content::BrowserContext* browser_context,
     const GURL& site) {
   std::string partition_id;
 
   // The partition ID for webview guest processes is the string value of its
   // 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();
   }
 
   DCHECK(IsValidStoragePartitionId(browser_context, partition_id));
   return partition_id;
 }
 
 bool ChromeContentBrowserClient::IsValidStoragePartitionId(
     content::BrowserContext* browser_context,
     const std::string& partition_id) {
@@ skipping 41 matching lines @@
       // the |partition_domain|. The |in_memory| and |partition_name| are only
       // used in guest schemes so they are cleared here.
       *partition_domain = site.host();
       *in_memory = false;
       partition_name->clear();
     }
     success = true;
   }
 #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;
   }
 
   // Assert that if |can_be_default| is false, the code above must have found a
   // non-default partition.  If this fails, the caller has a serious logic
   // error about which StoragePartition they expect to be in and it is not
   // safe to continue.
   CHECK(can_be_default || !partition_domain->empty());
 }
@@ skipping 1744 matching lines @@
       switches::kDisableWebRtcEncryption,
     };
     to_command_line->CopySwitchesFrom(from_command_line,
                                       kWebRtcDevSwitchNames,
                                       arraysize(kWebRtcDevSwitchNames));
   }
 }
 #endif  // defined(ENABLE_WEBRTC)
 
 }  // namespace chrome