Stop env from stringing where it should be passing None through unchanged.
Stop env from stringing where it should be passing None through unchanged.

None actually means 'delete this key from the environment', not 'please put the
string "None" in the env var'

R=blundell@chromium.org, pgervais@chromium.org
BUG=

Review URL: https://codereview.chromium.org/918283002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@294044 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build