Allow prodaccess based gsutil to also write into a bucket

The Telemetry workflow happily uses this gsutil to download artifacts, but they would also like to use the same workflow to upload artifacts.

I don't see a big harm in enabling read_write access for prodaccess based gsutil since its already more restrictive than the normal gsutil auth flow.  I could also be convinced to make this full_control, but that need has not arisen yet.

BUG= 341665

Review URL: https://codereview.chromium.org/148203012

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@259761 0039d316-1c4b-4281-b951-d872f2087c98

third_party/gsutil/plugins/sso_auth.py

@@ -24,7 +24,7 @@ STUBBY_CMD = """target: {
 target_credential: {
   type: OAUTH2_TOKEN
   oauth2_attributes: {
-    scope: 'https://www.googleapis.com/auth/devstorage.read_only'
+    scope: 'https://www.googleapis.com/auth/devstorage.read_write'
   }
 }"""