Implement sweep gradient layout

Add an FP that implements the sweep gradient effect and updates
SkSweepGradient to use the new system if possible.

Bug: skia:
Change-Id: I3f65da01afafae54c45848a6a78fd758f65eb4a6
Reviewed-on: https://skia-review.googlesource.com/148806
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
12 files changed