crosperf: fix the problem on machine assignment.

BUG=chromium:461628
TEST=it always reuses the image on the machine for test shown in bug.

Change-Id: I8106eaa31395db3ee95507b2a9cd7ed0439bc890
Reviewed-on: https://chrome-internal-review.googlesource.com/202667
Reviewed-by: Caroline Tice <cmtice@google.com>
Commit-Queue: Yunlian Jiang <yunlian@google.com>
Tested-by: Yunlian Jiang <yunlian@google.com>
1 file changed