Fix the problem that machine checksum are not checked before starting tests.

TEST=run an experiment that has machines with different checksum

Change-Id: I0da8244d168bdb3a0939a877c2ac5aa4c64a02f5
Reviewed-on: https://chrome-internal-review.googlesource.com/228371
Commit-Ready: Han Shen <shenhan@google.com>
Tested-by: Han Shen <shenhan@google.com>
Reviewed-by: Caroline Tice <cmtice@google.com>
1 file changed