add flag to stage virtual change in SkCanvas

Bug: skia:9984
Change-Id: I119ce10adcf14000d2f706f7c947f3c3f1da68a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083599Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746140}

skia/config/SkUserConfig.h

@@ -218,6 +218,10 @@ SK_API void SkDebugf_FileLine(const char* file,
 #define SK_SUPPORT_LEGACY_AAA_CHOICE
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_DRAWVERTS_VIRTUAL
+#define SK_SUPPORT_LEGACY_DRAWVERTS_VIRTUAL
+#endif
+
 // We're turning this off indefinitely,
 // until we can figure out some fundamental problems with its approach.
 //