See https://github.com/flutter/flutter/issues/6807

The Skia team says these APIs are outdated and should be removed. They
do not appear to have any clients.

Fixes https://github.com/flutter/flutter/issues/7202
Fixes https://github.com/flutter/flutter/issues/7204

This removes most (all?) of our dependency on base::RefCountedThreadSafe.

These parts don't depend on engine anymore. Also reformat them to match Google
C++ style.

Now that tonic doesn't depend on anything in //sky/engine anymore, we
can move the code to a location where its dependencies are clearer.

This patch paves the way to untangling a large amount of code from the engine.

For this, I added back my previous CustomDart attribute for customizing the
generated Dart code of an interface.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1162843003