Only remove CHROME_SANDBOX_ENV from env if it is present.

This will fix an error occuring on Windows.

(patch from csharp@chromium.org)

TBR=csharp

Review URL: https://codereview.chromium.org/17702004

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@208535 4ff67af0-8c30-449e-8e8b-ad334ec8d88c