Handle ninja_use_custom_environment_files case in GenerateEnvironmentFiles.
Handle ninja_use_custom_environment_files case in GenerateEnvironmentFiles.

(More fallout from r1612)

TBR=thakis@chromium.org

Review URL: https://codereview.chromium.org/13864006
old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google