Implements frame sniffer to watch iframe loading state.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index 56c85c52301c48da1082218a518e1f3b460c3027..aa011ac979e24c9e6085c915db653b882f142998 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -13170,6 +13170,9 @@ Keep your key file in a safe place. You will need it to create new versions of y
       <message name="IDS_LOGIN_FIX_CAPTIVE_PORTAL" desc="An link text to fix Internet connection (captive portal) in guest mode.">
         Start a Guest session
       </message>
+      <message name="IDS_LOGIN_MAYBE_CAPTIVE_PORTAL" desc="An offline message shown when we suggest that user may be behind the captive portal.">
+        Before signing in, please start a Guest session to activate the network or choose another one.

[Nikita (slow), 2011/08/31 15:29:15]

Might also use same message that is shown in offline help.
http://www.google.com/support/chromeos/bin/answer.py?hl=en&answer=1059250&ctx...

"Connected to a network, but still can't sign in? You might be on a network that
requires a web-based authentication. Start a guest browsing session (if
available) and see if a network log-in page appears. Once you're logged in to
the network and are able to visit different web pages, exit the guest session
and try signing in again."

[altimofeev, 2011/09/02 15:26:36]

Done.

+      </message>
       <message name="IDS_LOGIN_ERROR_CAPTIVE_PORTAL_NO_GUEST_MODE" desc="An error message shown when we suggest that user may be behind the captive portal but Guest mode is disabled.">
         Your password cannot be verified on this current network. Please select another network.
       </message>