Add functions to alpha-blend two colors and pick the more readable of two col...

app/gfx/color_utils.h

Index: app/gfx/color_utils.h
===================================================================
--- app/gfx/color_utils.h	(revision 25041)
+++ app/gfx/color_utils.h	(working copy)
@@ -52,9 +52,16 @@
 // this image.
 void BuildLumaHistogram(SkBitmap* bitmap, int histogram[256]);
 
-// Create a color from a base color and a specific alpha value.
-SkColor SetColorAlpha(SkColor c, SkAlpha alpha);
+// Returns a blend of the supplied colors, ranging from |background| (for
+// |alpha| == 0) to |foreground| (for |alpha| == 255).
+SkColor AlphaBlend(SkColor foreground, SkColor background, SkAlpha alpha);
 
+// Given two possible foreground colors, return the one that is more readable
+// over |background|.
+SkColor PickMoreReadableColor(SkColor foreground1,
+                              SkColor foreground2,
+                              SkColor background);
+
 // Gets a Windows system color as a SkColor
 SkColor GetSysSkColor(int which);