Let bot_update recipe api force usage of bot_update.

scripts/slave/recipe_modules/bot_update/example.py

Index: scripts/slave/recipe_modules/bot_update/example.py
diff --git a/scripts/slave/recipe_modules/bot_update/example.py b/scripts/slave/recipe_modules/bot_update/example.py
index 2d0c07a55e32fdd54ebf4b354e708ccb3ecf2ce3..d322300f600d8f253bd19e9f261d02eb96d6e05b 100644
--- a/scripts/slave/recipe_modules/bot_update/example.py
+++ b/scripts/slave/recipe_modules/bot_update/example.py
@@ -17,7 +17,8 @@ def GenSteps(api):
   soln.name = 'src'
   soln.url = 'svn://svn.chromium.org/chrome/trunk/src'
   api.gclient.c = src_cfg
-  yield api.bot_update.ensure_checkout()
+  force = True if api.properties.get('force') else False
+  yield api.bot_update.ensure_checkout(force=force)
 
 
 def GenTests(api):
@@ -34,3 +35,14 @@ def GenTests(api):
       patchset=654321,
       patch_url='http://src.chromium.org/foo/bar'
   )
+  yield api.test('forced') + api.properties(
+      mastername='experimental',
+      buildername='Experimental Builder',
+      slavename='somehost',
+      force=1
+  )
+  yield api.test('off') + api.properties(
+      mastername='experimental',
+      buildername='Experimental Builder',
+      slavename='somehost',
+  )