Switch Skia's sRGB color filter to share math with color space transforms

Less code and more consistent behavior.

Change-Id: Ide942100529a90bb2a2be3dc5893c57043052721
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2122478Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Cr-Commit-Position: refs/heads/master@{#753820}

skia/config/SkUserConfig.h

@@ -218,10 +218,6 @@ SK_API void SkDebugf_FileLine(const char* file,
 #define SK_SUPPORT_LEGACY_AAA_CHOICE
 #endif
 
-#ifndef SK_USE_LEGACY_SRGB_COLOR_FILTER
-#define SK_USE_LEGACY_SRGB_COLOR_FILTER
-#endif
-
 // We're turning this off indefinitely,
 // until we can figure out some fundamental problems with its approach.
 //