Make sure we use bytes and strings in the appropriate places:
* Call decode() when reading data from a file since we are then treating the
  data as a string.
* Make the GsutilsMock expectations use bytes rather than strings, as the
  non-mock version ultimately calls subprocess.Popen.communicate() in a mode
  that returns bytes, not strings.

Bug: 984182, 1007872
Change-Id: I7e03e404889d66f0f7cb77bbdbbfa9f7c5721657
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1859997
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>