The new GSUtil (or gs protocol, who knows) strips off the redundent x-goog-meta
string from the metadata key.  This CL compensates for that.

Also since we're on 4.7, we can use the faster gsutil stat instead of gsutil ls -L.

BUG=
TEST=ran download_from_google_storage against compiler_proxy.sha, works
NOTREECHECKS=true
NOTRY=true
R=dnj@chromium.org, pgervais@chromium.org

Review URL: https://codereview.chromium.org/809123003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293432 0039d316-1c4b-4281-b951-d872f2087c98