Add a script to bootstrap GN from source. For now only works on 64-bit Linux. BUG=336441 R=brettw@chromium.org Review URL: https://codereview.chromium.org/145133007 Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: f4821400355c5126dda788043ef6e148c067d160