This change:
- Sucks in gn binaries
- Sucks in a version of llvm that the GN build likes
- Adds tools/gn.py to invoke gn
- Adds a root BUILD.gn and .gn file
- Removes chrome boilerplate we don't need
- etc.
This lets us do a standalone build of the 'runtime'
target for x64, ia32, arm, arm64, mips, and the
simulators on Linux, and arm and arm64 on Android.
It does not include tcmalloc, and hasn't been tested
on Mac or Windows. That work and more cleanup of
chrome boilerplate will come in follow-up CLs.
R=johnmccutchan@google.com
Review URL: https://codereview.chromium.org/2350583002 .
