Actually disable load:tools:gmail

The decorator was applied to a base class which doesn't
actually cause anything to be disabled.

BUG=catapult#:3088

Review-Url: https://codereview.chromium.org/2575513002
Cr-Commit-Position: refs/heads/master@{#438149}

tools/perf/page_sets/system_health/loading_stories.py

@@ -279,7 +279,6 @@ class LoadDocsStory(_LoadingStory):
       'https://docs.google.com/document/d/1GvzDP-tTLmJ0myRhUAfTYWs3ZUFilUICg8psNHyccwQ/edit?usp=sharing')
 
 
-@decorators.Disabled('android')  # crbug.com.com/664505
 class _LoadGmailBaseStory(_LoadingStory):
   NAME = 'load:tools:gmail'
   URL = 'https://mail.google.com/mail/'
@@ -309,6 +308,8 @@ def _DidLoadDocument(self, action_runner):
     action_runner.WaitForJavaScriptCondition(
         'document.getElementById("loading").style.display === "none"')
 
+
+@decorators.Disabled('android')  # crbug.com.com/664505
 class LoadGmailMobileStory(_LoadGmailBaseStory):
   SUPPORTED_PLATFORMS = platforms.MOBILE_ONLY