Add MoltenVK support to Viewer.

Works with v1.0.17, will probably need updating for later revisions.

Bug: skia:8737
Change-Id: I9e42fad90656a88efa12625856019a8282ff39fd
Reviewed-on: https://skia-review.googlesource.com/c/191298
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index b194f2f..4dcfeb1 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2168,6 +2168,9 @@
         libs += [ "X11-xcb" ]
       } else if (is_win) {
         sources += [ "tools/sk_app/win/VulkanWindowContext_win.cpp" ]
+      } else if (is_mac) {
+        sources += [ "tools/sk_app/mac/VulkanWindowContext_mac.mm" ]
+        libs += [ "MetalKit.framework" ]
       }
     }