signin: force web signin flow initiated visits to accounts.google.com to their own process.

chrome/common/url_constants.h

Index: chrome/common/url_constants.h
diff --git a/chrome/common/url_constants.h b/chrome/common/url_constants.h
index 4441e2a986814cac7c0742047f27a7188f3b9051..384500f85ba1730a19ca7608ca59e2f481e11f11 100644
--- a/chrome/common/url_constants.h
+++ b/chrome/common/url_constants.h
@@ -413,6 +413,13 @@ extern const char kExtensionResourceScheme[];
 
 extern const char kChromeSearchScheme[];
 
+// This is used to distinguish URLs belonging to the special web signin flow
+// running in the special signin process from other URLs on the same domain.
+// We do not grant WebUI privilieges / bindings to this process or to URLs of
+// this scheme; enforcement of privileges is handled separately by
+// OneClickSigninHelper.
+extern const char kChromeSigninScheme[];
+
 #if defined(OS_CHROMEOS)
 extern const char kCrosScheme[];
 extern const char kDriveScheme[];