GN: ANGLE

Angle's existing GN files only work in Chrome, so I've written a new one.

This won't work on Windows, but our GN build doesn't work on Windows anyway.  So this CL is an attempt to get a ahead of that curve on ANGLE.  It looks large but fairly straightforward.

Now working on Linux:
  $ gn gen angle --args=skia_use_angle=true
  $ ninja -C angle
  $ angle/dm --config angle-gl --src gm -w dm-out

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2361983002

Review-Url: https://codereview.chromium.org/2361983002
4 files changed