Add best practice tip to identity API launchWebAuthFlow.

The tip is to suggest auth flows are initiated from app UI explaining
why. Without this, if a signed out user launches an app that immediately
initiates an interactive auth session, the first thing the user sees is
a chrome signin screen.

BUG=372279

Review URL: https://codereview.chromium.org/273423005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269963 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/api/identity.idl

@@ -87,6 +87,13 @@ namespace identity {
     // 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,