add flag for migrating fonthinting enum

See https://skia-review.googlesource.com/c/skia/+/168270

Change-Id: I8e782cbce2ddd8bd0fa77110cd718827e02342b9
Reviewed-on: https://chromium-review.googlesource.com/c/1318183
Commit-Queue: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605469}

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_NESTED_HINTINGENUM
+#define SK_SUPPORT_LEGACY_NESTED_HINTINGENUM
+#endif
+
 // Skia is enabling this feature soon. Chrome probably does
 // not want it for M64
 #ifndef SK_DISABLE_EXPLICIT_GPU_RESOURCE_ALLOCATION