Improve SkVerticesPriv ergonomics

Rather than two separate (partially overlapping) ways of accessing the
private portions of SkVertices, use a single privileged helper class
(similar to GrContextPriv).

Change-Id: I76b14b63088658ed8726719cce126577e5a52078
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/280601
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/src/utils/SkShadowUtils.cpp b/src/utils/SkShadowUtils.cpp
index 73005fe..9d272e1 100644
--- a/src/utils/SkShadowUtils.cpp
+++ b/src/utils/SkShadowUtils.cpp
@@ -589,7 +589,7 @@
 void SkBaseDevice::drawShadow(const SkPath& path, const SkDrawShadowRec& rec) {
     auto drawVertsProc = [this](const SkVertices* vertices, SkBlendMode mode, const SkPaint& paint,
                                 SkScalar tx, SkScalar ty, bool hasPerspective) {
-        if (SkVerticesPriv::VertexCount(vertices)) {
+        if (vertices->priv().vertexCount()) {
             // For perspective shadows we've already computed the shadow in world space,
             // and we can't translate it without changing it. Otherwise we concat the
             // change in translation from the cached version.