Add flag to stage API change

Flag is intended to be temporary, just long enough to change the
overrides in Chrome over to the new API.

See https://skia-review.googlesource.com/c/skia/+/338596

Change-Id: I0f7cc353ab0eec649b03ffa39e9ca1dba6d7b08f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2560217Reviewed-by: Aaron Krajeski <aaronhk@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
Cr-Commit-Position: refs/heads/master@{#831084}

skia/config/SkUserConfig.h

@@ -204,6 +204,10 @@ SK_API void SkDebugf_FileLine(const char* file,
 #   define SK_SUPPORT_LEGACY_ANISOTROPIC_MIPMAP_SCALE
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_CANVASMATRIX33
+#define SK_SUPPORT_LEGACY_CANVASMATRIX33
+#endif
+
 #ifndef SK_DISABLE_LEGACY_CONTEXT_FACTORIES
 #define SK_DISABLE_LEGACY_CONTEXT_FACTORIES
 #endif