Don't create newtab for gaia login.

BUG=417248
TEST=login_GaiaLogin

Review URL: https://codereview.chromium.org/600523006

Cr-Commit-Position: refs/heads/master@{#296470}

tools/telemetry/telemetry/core/backends/chrome/cros_browser_backend.py

@@ -232,7 +232,7 @@ class CrOSBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
     # Workaround for crbug.com/374462 - the bug doesn't manifest in the guest
     # session, which also starts with an open browser tab.
     retries = 3
-    while not self._is_guest:
+    while not self._is_guest and not self.browser_options.gaia_login:
       try:
         # Open a new window/tab.
         tab = self.tab_list_backend.New(timeout=30)