Make update_edge as a member function of SkAnalyticEdge

This is to prepare for future SkScan_DAAPath.cpp to use this member function
(https://skia-review.googlesource.com/c/19666/).

Bug: skia:
Change-Id: I5fb90ac67b80f8a69a712e7e697d68dcadba5d14
Reviewed-on: https://skia-review.googlesource.com/24180
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
4 files changed