add flag to guard font api changes

Change-Id: I977413e186201573e71de941d530c5fdf336ca0e
Reviewed-on: https://chromium-review.googlesource.com/c/1337820Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
Cr-Commit-Position: refs/heads/master@{#608547}

skia/config/SkUserConfig.h

@@ -145,6 +145,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_USE_LEGACY_DISTANCE_FIELDS
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_FONT_FLAGS
+#define SK_SUPPORT_LEGACY_FONT_FLAGS
+#endif
+
 // Skia is enabling this feature soon. Chrome probably does
 // not want it for M64
 #ifndef SK_DISABLE_EXPLICIT_GPU_RESOURCE_ALLOCATION