Revert of GN (patchset #12 id:220001 of https://codereview.chromium.org/2087593002/ )

Revert of GN (patchset #12 id:220001 of https://codereview.chromium.org/2087593002/ )

Reason for revert:
gclient not happy on some bots

Original issue's description:
> GN
>
> What we've got here is a little GN MVP.  It's lacking any knobs and doesn't yet build anything but libskia, zlib, libpng, and libjpeg-turbo.  I've been hopping back and forth between Linux at work and Mac at home.  These seem to be at least partially working, enough to build and run cmake/example.cpp.
>
> The xcode backend seems to work.  From here, we can start exploring how to handle other backends (cmake,Android make, Google3).  There are a couple things I want to try:
>   - add another backend like vs or xcode to GN directly
>   - intercept via a custom toolchain
>   - reverse from ninja -t commands
> That last option seems kind of fun.
>
> This tries to piggyback on Chrome's GN setup as much as possible.  Chrome's got quite a lot figured out, and we're basically required to do this if we want to have a single GN build system shareable by Chrome, our bots, and other clients.
>
> This pulls in some new DEPS:
>    - build: Chrome's GN configuration, and much more
>    - buildtools: hashes for gn binary, pulled via hooks
>    - tools/clang: hashes for Chrome's clang, pulled via hooks into third_party/llvm-build
> It additionally symlinks tools/gyp to third_party/externals/gyp.  GN pulls some stuff from tools/gyp on Mac.
>
> Have not yet tried building for Windows, Android, or iOS.
>
> BUG=skia:
> GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2087593002
>
> Committed: https://skia.googlesource.com/skia/+/1d8de594f126b9a80bd8f8fa2005e90faf3b5b17

TBR=bsalomon@google.com,mtklein@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review-Url: https://codereview.chromium.org/2088253002