Fix issue with offsetting polygon by 0 distance.

Need to output matching indices as well.

Bug: skia:13417, chromium:937412
Change-Id: I251a70eb07fda8b3a257d9f57ebdd9f20ad01b5a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/197160
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>

src/utils/SkPolyUtils.cpp

@@ -1164,6 +1164,7 @@ bool SkOffsetSimplePolygon(const SkPoint* inputPolygonVerts, int inputPolygonSiz
     if (SkScalarNearlyZero(offset)) {
         for (int i = 0; i < inputPolygonSize; ++i) {
             *offsetPolygon->push() = inputPolygonVerts[i];
+            *polygonIndices->push() = i;
         }
         return true;
     }