Revert of [tools/perf] Disable load:tools:drive system health story (patchset...

Revert of [tools/perf] Disable load:tools:drive system health story (patchset #1 id:1 of https://codereview.chromium.org/2848573002/ )

Reason for re-enabling the story:
The V8 CL that was causing the crash has been reverted in https://crrev.com/2852623002

Original issue's description:
> [tools/perf] Disable load:tools:drive system health story
>
> Story shows a high failure rate on many different perf bots.
>
> TBR=nednguyen@google.com
> BUG=715936
>
> Review-Url: https://codereview.chromium.org/2848573002
> Cr-Commit-Position: refs/heads/master@{#467640}
> Committed: https://chromium.googlesource.com/chromium/src/+/3721c3608a5aef73b1762e6906257c9d9f8e0d36

BUG=715936

Review-Url: https://codereview.chromium.org/2851703003
Cr-Commit-Position: refs/heads/master@{#467958}

tools/perf/page_sets/system_health/loading_stories.py

@@ -356,7 +356,6 @@ class LoadWeatherStory(_LoadingStory):
   TAGS = [story_tags.JAVASCRIPT_HEAVY]
 
 
-@decorators.Disabled('all')  # crbug.com/715936
 class LoadDriveStory(_LoadingStory):
   NAME = 'load:tools:drive'
   URL = 'https://drive.google.com/drive/my-drive'