Assigning to chanli@ for blamelist here: https://cs.chromium.org/chromium/infra/appengine/findit/waterfall/swarming_util.py?q=_GetOutputJsonHash+file:%5Einfra/appengine/findit/+package:%5Echromium$&l=423

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra/+/6c86729e36c8ef0e61ec7383099a50dad8a98ce0

commit 6c86729e36c8ef0e61ec7383099a50dad8a98ce0
Author: Chan <chanli@chromium.org>
Date: Wed Sep 27 00:48:02 2017

[Findit] Fix KeyError 'content'.

isolatedserver.retrieve API can return 2 kinds of results:

One contains a url, in this case Findit needs to send an additional http request to the url to get data;
Another contains content directly and Findit can directly process it and use it.

Previously I assumed case one can only happen at last when we get the step_log, but in the failing
example reported in  bug 768510 ,case one also happens when Findit tries to get output json hash.

Bug:  768510 
Change-Id: I1a61c1c2adfdde7b5584cca9af670c7561fb83ff
Reviewed-on: https://chromium-review.googlesource.com/685994
Commit-Queue: Chan Li <chanli@chromium.org>
Reviewed-by: Roberto Carrillo <robertocn@chromium.org>

[modify] https://crrev.com/6c86729e36c8ef0e61ec7383099a50dad8a98ce0/appengine/findit/waterfall/swarming_util.py
[modify] https://crrev.com/6c86729e36c8ef0e61ec7383099a50dad8a98ce0/appengine/findit/waterfall/test/swarming_util_test.py