add support for legacy yuv colorspace

First step in several to move YUV support into public interfaces
that are not in core, for consumption by chrome. See
https://skia-review.googlesource.com/c/skia/+/110880

R=fmalita@chromium.org

Change-Id: I237adcc03cd54402a2ca021dd91b821d0b2ad7db
Reviewed-on: https://chromium-review.googlesource.com/941661Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542518}

skia/config/SkUserConfig.h

@@ -213,6 +213,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_LEGACY_GPU_PIXEL_OPS
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_YUV_COLORSPACE
+#define SK_SUPPORT_LEGACY_YUV_COLORSPACE
+#endif
+
 // Max. verb count for paths rendered by the edge-AA tessellating path renderer.
 #define GR_AA_TESSELLATOR_MAX_VERB_COUNT 10