Allow variables in ENV_PASSTHRU to be used by cros_sdk itself.

Before patch:
  $ rm -rf /tmp/foo
  $ mkdir -p /tmp/foo
  $ CROS_CACHEDIR=/tmp/foo cros_sdk -- du -hsc /var/cache/chromeos-cache
24G     /var/cache/chromeos-cache
24G     total

After patch:
  $ rm -rf /tmp/foo
  $ mkdir -p /tmp/foo
  $ CROS_CACHEDIR=/tmp/foo cros_sdk -- du -hsc /var/cache/chromeos-cache
40K     /var/cache/chromeos-cache
40K     total

BUG=chromium-os:39695
TEST=CROS_CACHEDIR works now.
TEST=Also run a ton of trybots.

Change-Id: I95f808962f0cfe97ae56ac35a5e9014fdf2133b1
Reviewed-on: https://gerrit.chromium.org/gerrit/44849
Reviewed-by: Ryan Cui <rcui@chromium.org>
Reviewed-by: Brian Harring <ferringb@chromium.org>
Commit-Queue: David James <davidjames@chromium.org>
Tested-by: David James <davidjames@chromium.org>
diff --git a/scripts/cros_sdk.py b/scripts/cros_sdk.py
index e4a2291..93101d1 100644
--- a/scripts/cros_sdk.py
+++ b/scripts/cros_sdk.py
@@ -167,10 +167,10 @@
 def _SudoCommand():
   """Get the 'sudo' command, along with all needed environment variables."""
 
-  # Pass in the ENVIRONMENT_WHITELIST variable so that scripts in the chroot
-  # know what variables to pass through.
+  # Pass in the ENVIRONMENT_WHITELIST and ENV_PASSTHRU variables so that
+  # scripts in the chroot know what variables to pass through.
   cmd = ['sudo']
-  for key in constants.CHROOT_ENVIRONMENT_WHITELIST:
+  for key in constants.CHROOT_ENVIRONMENT_WHITELIST + constants.ENV_PASSTHRU:
     value = os.environ.get(key)
     if value is not None:
       cmd += ['%s=%s' % (key, value)]