chromeos: Correctly set the '--flash' arg for host-side tests.

It's already set for remote-side tests. After crrev.com/c/2283899, we
can set it for host-side tests too.

Bug: 1057152
Change-Id: I2e37d22fb24e928dff831d8383068fd8d1558edb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2285251Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785989}

build/chromeos/test_runner.py

@@ -718,6 +718,8 @@ def host_cmd(args, cmd_args):
     ]
   if args.verbose:
     cros_run_test_cmd.append('--debug')
+  if args.flash:
+    cros_run_test_cmd.append('--flash')
 
   if args.logs_dir:
     for log in SYSTEM_LOG_LOCATIONS: