[kitchen][auth] Add firebase auth scopes if enabled

Requests to firebase currently fail with an insufficient scopes error --
add the appropriate scope when firebase is enabled.

Bug: 904597

Change-Id: I9fd4759d4f0e62e951d5a4e9420174f71eca501a
Reviewed-on: https://chromium-review.googlesource.com/c/1409757
Commit-Queue: Julie Hockett <juliehockett@google.com>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#19988}
Cr-Mirrored-Commit: 9f897ad3e2d1a7a8db6170c6e2546d6bd760f431
diff --git a/go/src/infra/tools/kitchen/auth.go b/go/src/infra/tools/kitchen/auth.go
index 397c33c..4197813 100644
--- a/go/src/infra/tools/kitchen/auth.go
+++ b/go/src/infra/tools/kitchen/auth.go
@@ -147,7 +147,10 @@
 	// Construct authentication with default set of scopes to be used through out
 	// Kitchen.
 	authOpts := infraenv.DefaultAuthOptions()
-	authOpts.Scopes = OAuthScopes
+	authOpts.Scopes = append([]string(nil), OAuthScopes...)
+	if ac.EnableFirebaseAuth {
+		authOpts.Scopes = append(authOpts.Scopes, "https://www.googleapis.com/auth/firebase")
+	}
 	ac.authenticator = auth.NewAuthenticator(ac.ctx, auth.SilentLogin, authOpts)
 
 	// Figure out what email is associated with this account (if any).