Continue running if the upstream remote exists.

We already have code to check the git remotes to see if the upstream
exists, but apparently it does not always succeed (see
http://108.170.220.27:10115/builders/Housekeeper-PerCommit-AndroidRoll/builds/684/steps/Merge/logs/stdio
and
http://108.170.220.27:10115/builders/Housekeeper-PerCommit-AndroidRoll/builds/662/steps/Merge/logs/stdio
).

R=borenet@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/317823006
1 file changed