Don't allow rollback when we are enterprise enrolled.

As stated (and verified) by the device_policy protobuf, we can determine
whether a device is enterprise enrolled by checking if GetOwner is empty.
We use this knowledge to not allow rollback when powerwash is also requested (
the default).

As part of this CL I've figured out how to unittest Rollback and added tests
for both enterprise and non-enterprise rollback.

BUG=chromium:254829
TEST=Tested on both an enrolled and non-enrolled device. Verified only the
latter actually did a powerwash while the other aborted correctly. Also ran
new unittests

Change-Id: Idfe6bfef88819fe1bab7da6b31854faf7642c9ce
Reviewed-on: https://gerrit.chromium.org/gerrit/61645
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: Chris Sosa <sosa@chromium.org>
Tested-by: Chris Sosa <sosa@chromium.org>
diff --git a/utils.cc b/utils.cc
index 2bcc311..9d3bd26 100644
--- a/utils.cc
+++ b/utils.cc
@@ -572,7 +572,7 @@
 }
 
 const string BootKernelDevice(const std::string& boot_device) {
-  // Currntly this assumes the last digit of the boot device is
+  // Currently this assumes the last digit of the boot device is
   // 3, 5, or 7, and changes it to 2, 4, or 6, respectively, to
   // get the kernel device.
   string ret = boot_device;