If every file in a change has OWNERS, then the code would find
them fine. Similarly, if no files has OWNERS, then this code would
return an empty set just fine.

But if some files had OWNERS while others didn't, it would crash
when it tried to find an OWNER for file 'foo' while all the possible
OWNERS only provided coverage for file 'bar'.

This code purges the list of possible OWNERS as they become useless
for providing additional coverage, and returns whatever set we have
accumulated so far when the set of possible OWNERS becomes empty.

R=iannucci@chromium.org

Bug: 715062
Change-Id: I408601bd89379381db1cc7df56beed97ab3c27e6
Reviewed-on: https://chromium-review.googlesource.com/506239
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>