Add basic stacktrace handler using libunwind.

This means we will all have to apt-get install libunwind8-dev on Linux.  Mac comes with everything we need already.

BUG=skia:
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/343583005
13 files changed