Add an is_chromeos_chrome gn var, and use it for vm tests.

We're planning on adding more types of VM tests, so this flag should
come in handy in the future.

Bug: 876587
Change-Id: I0db628151ae2016b4fd891fdbd5064ee41b1720f
Reviewed-on: https://chromium-review.googlesource.com/1217073
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590094}

build/config/chromeos/rules.gni

@@ -9,6 +9,11 @@ declare_args() {
   cros_sdk_version = ""
 }
 
+# Ensure that if one is set, the other is as well.
+assert(cros_board == "" == (cros_sdk_version == ""))
+
+is_chromeos_chrome = cros_board != ""
+
 # Creates a script at $generated_script that can be used to launch a cros VM
 # and optionally run a test within it.
 # Args:

chromeos/BUILD.gn

@@ -801,7 +801,11 @@ test("chromeos_unittests") {
   ]
 }
 
-if (cros_board != "") {
+# The following definitions only apply when building for a CrOS board (ie: not
+# linux-chromeos). See the following for reference:
+# https://chromium.googlesource.com/chromium/src/+/master/docs/chromeos_build_instructions.md
+# https://chromium.googlesource.com/chromiumos/docs/+/master/simple_chrome_workflow.md
+if (is_chromeos_chrome) {
   generate_vm_runner_script("cros_vm_launcher") {
     generated_script = "$root_build_dir/bin/launch_cros_vm"