Rietveld: build sub-property also may be JSON
Rietveld: build sub-property also may be JSON

This CL is a work-around for corrupted TryJobResult entities. They have
"buildbucket" property which is a dict. Its "build" is JSON (should be dict).

R=sergiyb@chromium.org
BUG=532326

Review URL: https://codereview.chromium.org/1343293003 .

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra