Revert "remove SK_SUPPORT_LEGACY_SVG_ARC_TO"

This reverts commit 05ee0393.

Reason for revert: Sheriff reverting due to several paint-invalidation failing on the bot WebKit Mac 10.13 Retina.
See https://crbug.com/891230

Original change's description:
> remove SK_SUPPORT_LEGACY_SVG_ARC_TO
> 
> Change-Id: I330bdfe4dcbfed55a03385043e703af6fe070ffd
> Reviewed-on: https://chromium-review.googlesource.com/1254341
> Commit-Queue: Mike Reed <reed@chromium.org>
> Reviewed-by: Florin Malita <fmalita@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#595627}

TBR=fmalita@chromium.org,reed@chromium.org

Change-Id: Ica1ccdd43953772d40c70d22f3fc399df7dd56b1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 891230
Reviewed-on: https://chromium-review.googlesource.com/1256825Reviewed-by: Camille Lamy <clamy@chromium.org>
Commit-Queue: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595805}

skia/config/SkUserConfig.h

@@ -155,6 +155,11 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_SUPPORT_LEGACY_TILED_BITMAPS
 #endif
 
+// remove after rebaselining svg layout tests
+#ifndef SK_SUPPORT_LEGACY_SVG_ARC_TO
+#define SK_SUPPORT_LEGACY_SVG_ARC_TO
+#endif
+
 // Max. verb count for paths rendered by the edge-AA tessellating path renderer.
 #define GR_AA_TESSELLATOR_MAX_VERB_COUNT 100