Use chromium's build/gyp_environment.py to set default GYP variables
Use chromium's build/gyp_environment.py to set default GYP variables

This sets the default GYP_GENERATORS and also sets up
with windows toolchain installed in depot_tools.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4022.

Review URL: https://codereview.chromium.org/1420273009

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client