buildbucket: build sub-prop may be JSON
buildbucket: build sub-prop may be JSON

This is work-around for corrupted buildbot db.

TBR=sergiyb@chromium.org
BUG=532326

Review URL: https://codereview.chromium.org/1343313002 .

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@296705 0039d316-1c4b-4281-b951-d872f2087c98

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