depot_tools: Add a checkout_chromeos conditional var to gclient.

This is necessary for https://chromium-review.googlesource.com/c/chromium/src/+/882196.

Bug: 776987
Change-Id: Iec0a960a7d80f3ddbc0252073e30abc5a243f06c
Reviewed-on: https://chromium-review.googlesource.com/882318Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Benjamin Pastene <bpastene@chromium.org>

gclient.py

@@ -1264,6 +1264,7 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
     # Provide some built-in variables.
     result = {
         'checkout_android': 'android' in self.target_os,
+        'checkout_chromeos': 'chromeos' in self.target_os,
         'checkout_fuchsia': 'fuchsia' in self.target_os,
         'checkout_ios': 'ios' in self.target_os,
         'checkout_linux': 'unix' in self.target_os,