Pull in additional Chrome OS build support files.

Add an SConstruct file and a third_party/libevent/event.h
stub header needed to build libchrome on Chrome OS.

Also integrate additional Chrome OS configuration into
build/build_config.h.

BUG=chromium:521005

Change-Id: Ieb931aa482661694288c3b3f598b4642ffb9993d
3 files changed