Add best practice tip to identity API launchWebAuthFlow.

chrome/common/extensions/api/identity.idl

 // Copyright (c) 2013 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.
 
 // Use the <code>chrome.identity</code> API to get OAuth2 access tokens.
 namespace identity {
 
   dictionary TokenDetails {
     // Fetching a token may require the user to sign-in to Chrome, or
     // approve the application's requested scopes. If the interactive
@@ skipping 69 matching lines @@
     // Starts an auth flow at the specified URL.
     //
     // This method enables auth flows with non-Google identity
     // providers by launching a web view and navigating it to the
     // first URL in the provider's auth flow. When the provider
     // redirects to a URL matching the pattern
     // <code>https://&lt;app-id&gt;.chromiumapp.org/*</code>, the
     // window will close, and the final redirect URL will be passed to
     // the <var>callback</var> function.
     //
+    // For a good user experience it is important interactive auth flows are
+    // initiated by UI in your app explaining what the authorization is for.
+    // Failing to do this will cause your users to get authorization requests
+    // - or even Chrome sign in screens if they are not signed in - with
+    // no context. In particular, do not launch an interactive auth flow when
+    // your app is first launched.
+    //
     // |details| : WebAuth flow options.
     // |callback| : Called with the URL redirected back to your application.
     static void launchWebAuthFlow(WebAuthFlowDetails details,
                                   LaunchWebAuthFlowCallback callback);
 
     // Generates a redirect URL to be used in |launchWebAuthFlow|.
     //
     // The generated URLs match the pattern
     // <code>https://&lt;app-id&gt;.chromiumapp.org/*</code>.
     //
     // |path| : The path appended to the end of the generated URL.
     [nocompile] static DOMString getRedirectURL(optional DOMString path);
   };
 
   interface Events {
     // Fired when signin state changes for an account on the user's profile.
     static void onSignInChanged(AccountInfo account, boolean signedIn);
   };
 };