Add apply_gerrit_ref to bot_update api

recipe_modules/bot_update/example.py

Index: recipe_modules/bot_update/example.py
diff --git a/recipe_modules/bot_update/example.py b/recipe_modules/bot_update/example.py
index 10b4d118503885f672c70408220d18c61238110d..6020f9127c8aad0b53f4fa3f55776b5a1807e1c4 100644
--- a/recipe_modules/bot_update/example.py
+++ b/recipe_modules/bot_update/example.py
@@ -35,18 +35,25 @@ def RunSteps(api):
   suffix = api.properties.get('suffix')
   gerrit_no_reset = True if api.properties.get('gerrit_no_reset') else False
   gerrit_rebase_patch_ref = bool(api.properties.get('gerrit_rebase_patch_ref'))
-  api.bot_update.ensure_checkout(
-      force=force,
-      no_shallow=no_shallow,
-      patch=patch,
-      with_branch_heads=with_branch_heads,
-      output_manifest=output_manifest,
-      refs=refs, patch_oauth2=oauth2,
-      clobber=clobber,
-      root_solution_revision=root_solution_revision,
-      suffix=suffix,
-      gerrit_no_reset=gerrit_no_reset,
-      gerrit_rebase_patch_ref=gerrit_rebase_patch_ref)
+
+  if api.properties.get('test_apply_gerrit_ref'):
+    api.bot_update.apply_gerrit_ref(
+        root='/tmp/test/root',
+        gerrit_no_reset=gerrit_no_reset,
+        gerrit_rebase_patch_ref=gerrit_rebase_patch_ref)
+  else:
+    api.bot_update.ensure_checkout(
+        force=force,
+        no_shallow=no_shallow,
+        patch=patch,
+        with_branch_heads=with_branch_heads,
+        output_manifest=output_manifest,
+        refs=refs, patch_oauth2=oauth2,
+        clobber=clobber,
+        root_solution_revision=root_solution_revision,
+        suffix=suffix,
+        gerrit_no_reset=gerrit_no_reset,
+        gerrit_rebase_patch_ref=gerrit_rebase_patch_ref)
 
 
 def GenTests(api):
@@ -163,6 +170,12 @@ def GenTests(api):
       slavename='somehost',
       gerrit_rebase_patch_ref=True
   )
+  yield api.test('apply_gerrit_ref') + api.properties(
+      repository='chromium',
+      gerrit_rebase_patch_ref=True,
+      gerrit_no_reset=1,
+      test_apply_gerrit_ref=True,
+  )
   yield api.test('tryjob_v8') + api.properties(
       mastername='tryserver.chromium.linux',
       buildername='linux_rel',