The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/c691e438aa6449b48cdc318b1a762191f92d4aeb

commit c691e438aa6449b48cdc318b1a762191f92d4aeb
Author: Ningning Xia <nxia@chromium.org>
Date: Thu Aug 11 21:52:59 2016

Use git-cache for GCE builders.

Fix HostIsCIBuilder so --cache-dir can be applied to GCE builders.

BUG= chromium:637101 
TEST=unit_test

Change-Id: I46d522251335f4e236cadd4fba2ed8de225adf8c
Reviewed-on: https://chromium-review.googlesource.com/368352
Commit-Ready: Ningning Xia <nxia@chromium.org>
Tested-by: Ningning Xia <nxia@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>

[modify] https://crrev.com/c691e438aa6449b48cdc318b1a762191f92d4aeb/scripts/cbuildbot.py
[modify] https://crrev.com/c691e438aa6449b48cdc318b1a762191f92d4aeb/lib/cros_build_lib.py
[modify] https://crrev.com/c691e438aa6449b48cdc318b1a762191f92d4aeb/lib/cros_build_lib_unittest.py
[modify] https://crrev.com/c691e438aa6449b48cdc318b1a762191f92d4aeb/cbuildbot/cbuildbot_unittest.py
[modify] https://crrev.com/c691e438aa6449b48cdc318b1a762191f92d4aeb/cbuildbot/constants.py