The purpose is to allow mixed usage of Rietveld/Gerrit in the same repo.
There, a branch can use Gerrit, even though repo's default is Rietveld.
The only remaining repo-wide check GetIsGerrit is to determine codereview
for a branch which has no uploads yet.

This also simplifies future test expectations.

R=andybons@chromium.org,bauerb@chromium.org
BUG=598681

Review URL: https://codereview.chromium.org/1851663002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299603 0039d316-1c4b-4281-b951-d872f2087c98