Use gclient's git_cache setting in bot_update/api.py

recipe_modules/gclient/example.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 DEPS = [
   'gclient',
   'recipe_engine/path',
   'recipe_engine/properties',
 ]
 
@@ skipping 40 matching lines @@
   'webports',
   'webrtc_test_resources',
   'with_branch_heads',
 ]
 
 
 def RunSteps(api):
   for config_name in TEST_CONFIGS:
     api.gclient.make_config(config_name)
 
-  src_cfg = api.gclient.make_config(GIT_MODE=True)
+  src_cfg = api.gclient.make_config(GIT_MODE=True, CACHE_DIR='[ROOT]/git_cache')
   soln = src_cfg.solutions.add()
   soln.name = 'src'
   soln.url = 'https://chromium.googlesource.com/chromium/src.git'
   soln.revision = api.properties.get('revision')
   src_cfg.parent_got_revision_mapping['parent_got_revision'] = 'got_revision'
   api.gclient.c = src_cfg
   api.gclient.checkout()
 
   api.gclient.spec_alias = 'WebKit'
   bl_cfg = api.gclient.make_config()
@@ skipping 19 matching lines @@
 
   assert not api.gclient.is_blink_mode
 
 
 def GenTests(api):
   yield api.test('basic')
 
   yield api.test('revision') + api.properties(revision='abc')
 
   yield api.test('tryserver') + api.properties.tryserver()