Remove unused function from color_utils.

ui/gfx/color_utils.h

Index: ui/gfx/color_utils.h
diff --git a/ui/gfx/color_utils.h b/ui/gfx/color_utils.h
index 1ab38c079bfd7ba593889bef8c189e982559ee7c..1beb7c6b12a672261d3cb1ace5ba6ef214c7b684 100644
--- a/ui/gfx/color_utils.h
+++ b/ui/gfx/color_utils.h
@@ -51,11 +51,6 @@ bool IsColorCloseToTransparent(SkAlpha alpha);
 // Determine if a color is near grey.
 bool IsColorCloseToGrey(int r, int g, int b);
 
-// Gets a color representing a bitmap. The definition of "representing" is the
-// average color in the bitmap. The color returned is modified to have the
-// specified alpha.
-SkColor GetAverageColorOfFavicon(SkBitmap* bitmap, SkAlpha alpha);
-
 // Builds a histogram based on the Y' of the Y'UV representation of
 // this image.
 UI_EXPORT void BuildLumaHistogram(const SkBitmap& bitmap, int histogram[256]);