This is for transitioning us out of the horribly outdated gsutil 3.4 thats
currently residing in depot_tools.  This script:
1. Downloads GSUtil, pinned to a version (4.6 by default).
2. Executes GSUtil with the given parameters
3. (TODO) Runs the fallback GSUtil if #2 fails.


The transition plan is:
1. Set the fallback as default, making this script a no-op
2. Modify recipes (main consumer of gsutil) to pass in --force_version 4.6
3. Switch 4.6 as the default for gsutil.py
4. Make fallback failure a loud red failure
5. Remove depot_tools/third_party/gsutil

BUG=434422

Review URL: https://codereview.chromium.org/742173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293320 0039d316-1c4b-4281-b951-d872f2087c98