OOBE sign in fixes for enterprise_PowerManagement

Debugging some tests that were unblocked by codereview.chromium.org/1084793002, I found that waiting for the password field to disappear sometimes fails. We need to wait for the OOBE page to disappear before continuing to the test logic.

BUG=480571

Review URL: https://codereview.chromium.org/1063853008

Cr-Commit-Position: refs/heads/master@{#326915}

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

@@ -91,7 +91,11 @@ class Oobe(web_contents.WebContents):
   def _NavigateWebViewLogin(self, username, password):
     """Logs into the webview-based GAIA screen"""
     self._NavigateWebViewEntry('identifierId', username)
+    self._GaiaWebViewContext().WaitForJavaScriptExpression(
+        "document.getElementById('identifierId') == null", 20)
+
     self._NavigateWebViewEntry('password', password)
+    util.WaitFor(lambda: self._GaiaWebViewContext() == None, 20)
 
   def _NavigateWebViewEntry(self, field, value):
     self._WaitForField(field)
@@ -101,8 +105,6 @@ class Oobe(web_contents.WebContents):
        document.getElementById('%s').value='%s';
        document.getElementById('next').click()"""
            % (field, value))
-    gaia_webview_context.WaitForJavaScriptExpression(
-        "document.getElementById('%s') == null" % field, 20)
 
   def _WaitForField(self, field_id):
     gaia_webview_context = util.WaitFor(self._GaiaWebViewContext, 5)