add flag for legacy SkPath arcTo behavior

Skia upstream change keeps rounded rectangles
constructed from SVG convex. This alters
some existing SVG layout results. Disable
the new behavior pending the rebaseline.

TBR=djsollen@chromium.org

Bug: b:69622768
Change-Id: Ia32b9cffff4a011bc835eb014f3943e735d6775e
Reviewed-on: https://chromium-review.googlesource.com/811840Reviewed-by: Cary Clark <caryclark@chromium.org>
Commit-Queue: Cary Clark <caryclark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522229}

skia/config/SkUserConfig.h

@@ -232,6 +232,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_COLOR_SPACE_XFORM_LEGACY_PIPELINE
 #endif
 
+// remove after rebaselining svg layout tests
+#ifndef SK_SUPPORT_LEGACY_SVG_ARC_TO
+#define SK_SUPPORT_LEGACY_SVG_ARC_TO
+#endif
 
 ///////////////////////// Imported from BUILD.gn and skia_common.gypi