Calculate line breaks using Minikin

This patch moves the calculation of line breaks for StaticLayout into
the new LineBreaker class provided by Minikin. This specific patch
should preserve existing functionality, but perhaps performance is
better, and the movement opens the door to much more sophisticated
line-breaking.

Change-Id: Iafccb9da4e3559bbeaeb2c7c85f86ddfd8ae2fa1