Android: allow host OS to be passed to envsetup.

build/android/envsetup.sh

 #!/bin/bash
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Sets up environment for building Chromium on Android.  It can either be
 # compiled with the Android tree or using the Android SDK/NDK. To build with
 # NDK/SDK: ". build/android/envsetup.sh".  Environment variable
 # ANDROID_SDK_BUILD=1 will then be defined and used in the rest of the setup to
 # specifiy build type.
@@ skipping 38 matching lines @@
       echo "WARNING: 32-bit host build enabled. Here be dragons!"
       host_arch=x86
     fi
     ;;
   *)
     echo "ERROR: Unsupported host architecture (${host_arch})."
     echo "Try running this script on a Linux/x86_64 machine instead."
     return 1
 esac
 
-host_os=$(uname -s | sed -e 's/Linux/linux/;s/Darwin/mac/')
-
 case "${host_os}" in
   "linux")
     toolchain_dir="linux-${host_arch}"
     ;;
   "mac")
     toolchain_dir="darwin-${host_arch}"
     ;;
   *)
     echo "Host platform ${host_os} is not supported" >& 2
     return 1
@@ skipping 84 matching lines @@
   # This is just a simple wrapper of gyp_chromium, please don't add anything
   # in this function.
   echo "GYP_GENERATORS set to '$GYP_GENERATORS'"
   (
     "${CHROME_SRC}/build/gyp_chromium" --depth="${CHROME_SRC}" --check "$@"
   )
 }
 
 # FLOCK needs to be null on system that has no flock
 which flock > /dev/null || export FLOCK=