Reland https://skia-review.googlesource.com/c/6091/

Reland https://skia-review.googlesource.com/c/6091/

The only difference is that we now put the guard flag SK_SUPPORT_LEGACY_AAA in
SkUserConfig.h instead of SkScan.h. Previously, SkAnalyticEdge.cpp doesn't get
that flag from SkScan.h and that caused many problems.

BUG=skia:
TBR=reed@google.com,caryclark@google.com

Change-Id: I7b89d3cb64ad71715101d2a5e8e77be3a8a6fa16
Reviewed-on: https://skia-review.googlesource.com/6972
Reviewed-by: Yuqian Li <liyuqian@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>