Proof of concept of ignoring x-frame-options for sign-in webui.

chrome/browser/chrome_content_browser_client.cc

Index: chrome/browser/chrome_content_browser_client.cc
===================================================================
--- chrome/browser/chrome_content_browser_client.cc	(revision 248093)
+++ chrome/browser/chrome_content_browser_client.cc	(working copy)
@@ -736,8 +736,12 @@
 
   // 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))
+  if (site.SchemeIs(content::kGuestScheme)) {
     partition_id = site.spec();
+  }  else if (site.SchemeIs(chrome::kChromeUIScheme) &&
+             site.host() == chrome::kChromeSigninHost) {
+    partition_id = site.spec();
+  }
 
   DCHECK(IsValidStoragePartitionId(browser_context, partition_id));
   return partition_id;
@@ -801,6 +805,9 @@
       *in_memory = false;
       partition_name->clear();
     }
+  } else if (site.SchemeIs(chrome::kChromeUIScheme) &&
+             site.host() == chrome::kChromeSigninHost) {
+    *partition_domain = chrome::kChromeSigninHost;
   }
 
   // Assert that if |can_be_default| is false, the code above must have found a