Default GYP to using ninja.
Default GYP to using ninja.

Currently all the bots use ninja, this will make local checkouts do the same.

BUG=none

Review URL: https://codereview.chromium.org/922123003

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