chromeos: Add the --flash arg to all HW tests via GN test template.

Bug: 1132373
Change-Id: I2490fcea413a338cd8ecebc19c4ab25680a53e7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2432127Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810850}

build/config/chromeos/rules.gni

@@ -153,6 +153,8 @@ template("generate_runner_script") {
 
     if (cros_is_vm) {
       args += [ "--use-vm" ]
+    } else {
+      args += [ "--flash" ]
     }
 
     # If we have public Chromium builds, use public Chromium OS images when