buildbucket: get_triggering_service works > once
buildbucket: get_triggering_service works > once

buildbucket's get_triggering_service worked only once. This CL fixes
that

TBR=stip@chromium.org
BUG=449218

Review URL: https://codereview.chromium.org/967223003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@294336 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build