Sometimes the spec passed in via parameters also have got_revision_mapping that
we want to use.

BUG=590788

Review URL: https://codereview.chromium.org/1748143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299015 0039d316-1c4b-4281-b951-d872f2087c98