[autotest] bug reporter finds credentials in creds/ dir

Use the util method to get full path of credentials
that lives in creds/ dir.

CQ-DEPEND=CL:*241179
TEST=File a bug with the reporter.
BUG=chromium:349505,chromium:567882

Change-Id: Ida17d3ab1c39118069c31a7401985a3353f61079
Reviewed-on: https://chromium-review.googlesource.com/317382
Commit-Ready: Fang Deng <fdeng@chromium.org>
Tested-by: Fang Deng <fdeng@chromium.org>
Reviewed-by: Dan Shi <dshi@google.com>
2 files changed