GN build

```
switch from gyp to GN build.
1. r991 added BUILD.gn, supporting chrome builds of libyuv via GN.
2. r1317 added setup support to allow stand alone GN builds

Next steps are switching all gyp build scripts to GN, improving functionality 
and documenting how to build with GN

```

Original issue reported on code.google.com by `fbarch...@google.com` on 10 Mar 2015 at 4:52
