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

chrome/common/url_constants.cc

Index: chrome/common/url_constants.cc
diff --git a/chrome/common/url_constants.cc b/chrome/common/url_constants.cc
index 2341c08b9b8b6b660fb9c4f8f521026fb98b3ad3..83de1b73648dfb699298e02b9b17448f70ae136e 100644
--- a/chrome/common/url_constants.cc
+++ b/chrome/common/url_constants.cc
@@ -519,6 +519,8 @@ const char kExtensionResourceScheme[] = "chrome-extension-resource";
 
 const char kChromeSearchScheme[] = "chrome-search";
 
+const char kChromeSigninScheme[] = "chrome-signin";
+
 // Google SafeSearch query parameters.
 const char kSafeSearchSafeParameter[] = "safe=active";
 const char kSafeSearchSsuiParameter[] = "ssui=on";