Add support for the project 'upstream' attribute  in manifests.
Add support for the project 'upstream' attribute  in manifests.

'repo manifest -r' adds an 'upstream' attribute to the project tag for
the manifests it generates.  We can use the attribute to get a valid
branch instead of a sha1 for these types of manifests.

BUG=chromium:249566
TEST=Manual, (pending) trybots.

Change-Id: I34daa2b59aedaa8ea08f119a252aa4397710769b
Reviewed-on: https://gerrit.chromium.org/gerrit/59575
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>

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