Add guard to allow changing Skia's YUV -> RGB matrices

The fixes to the Rec.601 and Rec.709 matrices affect a handful
of layout tests.

See also: https://skia-review.googlesource.com/c/skia/+/251771/

Bug: skia:9543
Change-Id: Ia6ffa7742e945348bc7af68d6c2214fd66a50a96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1890956
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Cr-Commit-Position: refs/heads/master@{#711225}

skia/config/SkUserConfig.h

@@ -222,6 +222,10 @@ SK_API void SkDebugf_FileLine(const char* file,
 #define SK_SUPPORT_LEGACY_AAA_CHOICE
 #endif
 
+#ifndef SK_LEGACY_YUV_MATRICES
+#define SK_LEGACY_YUV_MATRICES
+#endif
+
 // We're turning this off indefinitely,
 // until we can figure out some fundamental problems with its approach.
 //