Allow gclient solution URLs to be |None|.

This is supported by gclient and has the effect of treating the solution
as a purely local, standalone DEPS (i.e. not as a source repo to
checkout or process in any way). It also prevents the solution from
being added to the output of 'gclient flatten'.

R=agable@chromium.org, phajdan.jr@chromium.org

Change-Id: Ie11246ec7a74097b96ea6a26983a63b3fb3123a5
Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: build
Reviewed-on: https://chromium-review.googlesource.com/698271
Commit-Queue: Michael Moss <mmoss@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>

recipes/recipe_modules/gclient/config.py

@@ -18,7 +18,7 @@ def BaseConfig(USE_MIRROR=True, CACHE_DIR=None,
     solutions = ConfigList(
       lambda: ConfigGroup(
         name = Single(basestring),
-        url = Single(basestring),
+        url = Single((basestring, type(None)), empty_val=''),
         deps_file = Single(basestring, empty_val='.DEPS.git', required=False,
                            hidden=False),
         managed = Single(bool, empty_val=True, required=False, hidden=False),