add a guard for Win32 SkJumper backend

Anticipating ~140 layout test diffs:
  https://skia-review.googlesource.com/c/20968/13

Change-Id: Ia5de315a6b04249ad5042add8f193da7c4db7269
Reviewed-on: https://chromium-review.googlesource.com/574871Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487220}

skia/config/SkUserConfig.h

@@ -216,6 +216,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_SUPPORT_LEGACY_TILED_BITMAPS
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_WIN32_JUMPER
+#define SK_SUPPORT_LEGACY_WIN32_JUMPER
+#endif
+
 #ifndef SK_SUPPORT_NONSTD_BLENDMODES
 #define SK_SUPPORT_NONSTD_BLENDMODES
 #endif