cros_build_lib: convert CommandResult to CompletedProcess

Since CommandResult is just CompletedProcess at this point, switch
away from that to better match subprocess APIs.

BUG=b:187789262
TEST=CQ passes

Change-Id: I888ef8c27d8a57b6461deade4ff57846c6e8b04e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3818209
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
diff --git a/scripts/cros_sdk_unittest.py b/scripts/cros_sdk_unittest.py
index 5e43307..5a80597 100644
--- a/scripts/cros_sdk_unittest.py
+++ b/scripts/cros_sdk_unittest.py
@@ -67,13 +67,13 @@
     m = self.PatchObject(retry_util, 'RunCurl')
     with self.assertRaises(ValueError):
       cros_sdk.FetchRemoteTarballs(self.tempdir, [])
-    m.return_value = cros_build_lib.CommandResult(stdout=b'Foo: bar\n')
+    m.return_value = cros_build_lib.CompletedProcess(stdout=b'Foo: bar\n')
     with self.assertRaises(ValueError):
       cros_sdk.FetchRemoteTarballs(self.tempdir, ['gs://x.tar'])
 
   def testFetchRemoteTarballsSuccess(self):
     """Test FetchRemoteTarballs with a successful download."""
-    curl = cros_build_lib.CommandResult(stdout=(
+    curl = cros_build_lib.CompletedProcess(stdout=(
         b'HTTP/1.0 200\n'
         b'Foo: bar\n'
         b'Content-Length: 100\n'