[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>
diff --git a/site_utils/phapi_lib.py b/site_utils/phapi_lib.py
index 7f05560..2dd1e0b 100644
--- a/site_utils/phapi_lib.py
+++ b/site_utils/phapi_lib.py
@@ -89,6 +89,9 @@
         if apiclient_build is None:
             raise ProjectHostingApiException('Cannot get apiclient library.')
 
+        if not oauth_credentials:
+            raise ProjectHostingApiException('No oauth_credentials is provided.')
+
         storage = oauth_client_fileio.Storage(oauth_credentials)
         credentials = storage.get()
         if credentials is None or credentials.invalid: