gerrit_util: Add GCE metadata server auth.
gerrit_util: Add GCE metadata server auth.

BUG=chromium:532318
TEST=None

Review URL: https://codereview.chromium.org/1350673004

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@296930 0039d316-1c4b-4281-b951-d872f2087c98

aleonliao/depot_tools,azureplus/chromium_depot_tools,azureplus/chromium_depot_tools,Midrya/chromium,azureplus/chromium_depot_tools,primiano/depot_tools,aleonliao/depot_tools,primiano/depot_tools,duongbaoduy/gtools,disigma/depot_tools,disigma/depot_tools,hsharsha/depot_tools,duongbaoduy/gtools,Midrya/chromium,primiano/depot_tools,duanwujie/depot_tools,CoherentLabs/depot_tools,hsharsha/depot_tools,Midrya/chromium,duanwujie/depot_tools,CoherentLabs/depot_tools,hsharsha/depot_tools,duanwujie/depot_tools,aleonliao/depot_tools,duongbaoduy/gtools,disigma/depot_tools