Upload LATEST to base_upload_url instead of upload_url.
Upload LATEST to base_upload_url instead of upload_url.

The LATEST files are currently uploaded to the wrong directory.
They need to be stored under the base_upload_url (the parent
directory of the upload url), so that users can find out what
build is the latest.

BUG=chromium:314773
TEST=unit tests
TEST=trybot run

Change-Id: I42786991d2a200c2aee0e4cf40719bb875752b36
Reviewed-on: https://chromium-review.googlesource.com/175849
Reviewed-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite