* Added drawArc to Canvas.

Added a way to call SkCanvas::drawArc from Dart. This method is
optimized for the arc case and should be faster that drawing paths.

* Removed implementation details from docs.

* Added more details to the docs.

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

Also, switch over to //lib/tonic's typed data classes.

This patch moves the remainder of painting and compositing into their
new home (and updates their style to match Google C++ Style).