Add arcs as a specialized geometry to GrShape.

BUG: skia:7794

Change-Id: I484693711f48e55631732a0f4ee97e2848dec89d
Reviewed-on: https://skia-review.googlesource.com/122900
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
7 files changed