Add force_sync option to chromite.checkout.

scripts/slave/recipe_modules/chromite/api.py

Index: scripts/slave/recipe_modules/chromite/api.py
diff --git a/scripts/slave/recipe_modules/chromite/api.py b/scripts/slave/recipe_modules/chromite/api.py
index 91abefab85b2146ed1f3b12624733f027b7c2729..0d6030005ac3fa39735241f2bcd53ae3f6ca61b3 100644
--- a/scripts/slave/recipe_modules/chromite/api.py
+++ b/scripts/slave/recipe_modules/chromite/api.py
@@ -203,12 +203,15 @@ class ChromiteApi(recipe_api.RecipeApi):
     soln.revision = '%s:' % (self.c.chromite_branch,)
     return cfg
 
-  def checkout(self, manifest_url=None, repo_url=None):
+  def checkout(self, manifest_url=None, repo_url=None, force_sync=False):
     manifest_url = manifest_url or self.manifest_url
     repo_url = repo_url or self.repo_url
 
     self.m.repo.init(manifest_url, '--repo-url', repo_url)
-    self.m.repo.sync()
+    args = []
+    if force_sync:
+      args.append('--force-sync')
+    self.m.repo.sync(*args)
 
   @property
   def using_old_chromite_layout(self):