Add 'chromeos' as one of the DEPS_OS_CHOICES.

This makes 'checkout_chromeos' conditions visible to '--deps=all'
processing.

R=ehmaldonado@google.com

Bug: 928829
Change-Id: I1fe3d73117203d549c7039f7621eecadd3ac344e
Reviewed-on: https://chromium-review.googlesource.com/c/1454849Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

gclient.py

@@ -1280,6 +1280,7 @@ class GClient(GitDependency):
     "android": "android",
     "ios": "ios",
     "fuchsia": "fuchsia",
+    "chromeos": "chromeos",
   }
 
   DEFAULT_CLIENT_FILE_TEXT = ("""\