This is needed for https://chromiumcodereview.appspot.com/12042069/
Which uses gsutil to download objects from Google Storage based on SHA1 sums

Continuation of: https://chromiumcodereview.appspot.com/12317103/
Rietveld didn't like a giant CL with all of gsutil (kept crashing on upload),
The CL is being split into three parts

Related:
https://chromiumcodereview.appspot.com/12755026/ (gsutil/boto)
https://chromiumcodereview.appspot.com/12685010/ (gsutil/gslib)

BUG=

Review URL: https://codereview.chromium.org/12685009

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@188896 0039d316-1c4b-4281-b951-d872f2087c98