Add tools to build Dart with Chromium's sysroot.

tools/download_chromium_sysroot.sh

Index: tools/download_chromium_sysroot.sh
diff --git a/tools/download_chromium_sysroot.sh b/tools/download_chromium_sysroot.sh
new file mode 100755
index 0000000000000000000000000000000000000000..75b3744e15dec5366d4b4bebbcdbe1dce6eb7d91
--- /dev/null
+++ b/tools/download_chromium_sysroot.sh
@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+
+# Downloads the ia32 and x64 Debian wheezy sysroot that chromium uses,
+# Only tested and used on Ubuntu trusty linux. Used to keep glibc version low.
+# Creates directories called "build" and "tools" in the current directory.
+# After running this, source set_ia32_sysroot.sh or set_x64_sysroot.sh, in 
+# the same working directory, to set the compilation environment variables.
+# Sourcing a script means running the script with a '.', so that it runs
+# in the current shell, not a subshell, as in:
+#   . sdk/tools/set_ia32_sysroot.sh
+
+git clone https://chromium.googlesource.com/chromium/src/build

[Søren Gjesse, 2016/08/10 13:44:00]

Do we want to pin this at a specific Git hash, to make sure we will not be hit
by breaking changes?

[Bill Hesse, 2016/08/10 14:04:26]

I don't think so.  I think we should expect changes to happen rarely, and adapt
to them when they happen.  This script should mainly be run by hand anyway - I
don't plan to run it on the buildbots automatically.

+mkdir tools
+cd tools
+git clone https://chromium.googlesource.com/external/gyp
+cd ..
+
+build/linux/sysroot_scripts/install-sysroot.py --arch i386
+build/linux/sysroot_scripts/install-sysroot.py --arch amd64