Add utils to better quantize grayscale values to three bit indices while
compressing coverage masks.

Signed-off-by: Pavel Krajcevski <pavel@cs.unc.edu>

BUG=skia:

Review URL: https://codereview.chromium.org/669243003
diff --git a/gyp/utils.gypi b/gyp/utils.gypi
index e62d287..7d40a8b 100644
--- a/gyp/utils.gypi
+++ b/gyp/utils.gypi
@@ -88,6 +88,7 @@
         '<(skia_src_path)/utils/SkRTConf.cpp',
         '<(skia_src_path)/utils/SkTextureCompressor.cpp',
         '<(skia_src_path)/utils/SkTextureCompressor.h',
+        '<(skia_src_path)/utils/SkTextureCompressor_Utils.h',
         '<(skia_src_path)/utils/SkTextureCompressor_ASTC.cpp',
         '<(skia_src_path)/utils/SkTextureCompressor_ASTC.h',
         '<(skia_src_path)/utils/SkTextureCompressor_Blitter.h',