This patch improves script run itemization and also exposes metrics
and bounds for layouts. In addition, there is a fair amount of internal
cleanup, including ref counting, and making the MinikinFont abstraction
strong enough to support both FreeType and Skia implementations. There
is also a sample implementation using Skia, in the sample directory.

As part of its functionality, his patch measures the bounds of the
layout and gives access through Layout::GetBounds().  The corresponding
method is not implemented in the FreeType-only implementation of
MinikinFont, so that will probably have to be fixed.

Change-Id: Ib1a3fe9d7c90519ac651fb4aa957848e4bb758ec