add SkVptr()

I was wondering how feasible using this to make downcasts safe would be.
These tests would need to build and pass on all our bots, at least.

Change-Id: I1753ba58841bf6c17d6ac3af7374518356e1bb05
Reviewed-on: https://skia-review.googlesource.com/81180
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/gn/tests.gni b/gn/tests.gni
index 8c08d16..30f6190 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -274,6 +274,7 @@
   "$_tests/VkHeapTests.cpp",
   "$_tests/VkUploadPixelsTests.cpp",
   "$_tests/VkWrapTests.cpp",
+  "$_tests/VptrTest.cpp",
   "$_tests/WindowRectanglesTest.cpp",
   "$_tests/WritePixelsTest.cpp",
   "$_tests/Writer32Test.cpp",