auto_updater_transfer: Fix issue with GetPayloadSize()

In the fallback for GetPayloadSize(), it is not specified that the
output of the curl command should go to stdout. So nothing goes to
output to be parsed. This CL fixes that.

BUG=None
TEST=run_pytest
TEST=test_that --args="target_release=13280.0.0 target_payload_uri='gs://chromeos-releases/dev-channel/reef/13290.0.0/payloads/chromeos_13290.0.0_reef_dev-channel_full_test.bin-gvswkobwg43gf4byzyw75rj4io3sl5y5' source_release=13280.0.0 source_payload_uri='gs://chromeos-releases/dev-channel/reef/13290.0.0/payloads/chromeos_13290.0.0_reef_dev-channel_full_test.bin-gvswkobwg43gf4byzyw75rj4io3sl5y5' update_type=full" chromeos6-row4-rack9-host3.cros.corp.google.com autoupdate_EndToEndTest

Change-Id: I6459ad38637ffa875b8f5f5ff2facb28229c6856
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2257812
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sanika Kulkarni <sanikak@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
diff --git a/lib/auto_updater_transfer.py b/lib/auto_updater_transfer.py
index b1ce21c..0e8ef8c 100644
--- a/lib/auto_updater_transfer.py
+++ b/lib/auto_updater_transfer.py
@@ -589,7 +589,7 @@
       # TODO(crbug.com/1059008): Fallback in case the try block above fails.
       # Should be removed once reliable.
       try:
-        proc = retry_util.RunCurl(cmd[1:])
+        proc = retry_util.RunCurl(cmd[1:], stdout=True)
       except cros_build_lib.RunCommandError as e:
         raise ChromiumOSTransferError('Unable to get payload size: %s' % e)