crosperf: fix the label option chrome_src

This fixes the error when chrome_src is given to label.

BUG=None
TEST=run a telemetry test with chrome_src is specified.

Change-Id: I5f97c731bec3bcb42c1172490d765f8921d30dc0
Reviewed-on: https://gerrit-int.chromium.org/42197
Reviewed-by: Caroline  Tice <cmtice@google.com>
Commit-Queue: Yunlian Jiang <yunlian@google.com>
Tested-by: Yunlian Jiang <yunlian@google.com>
1 file changed