views: refactor away PlatformStyle::BackgroundColorForMdButton

ui/gfx/color_utils.cc

Index: ui/gfx/color_utils.cc
diff --git a/ui/gfx/color_utils.cc b/ui/gfx/color_utils.cc
index 59ecedc5dac7dcb9121c83ef1a54b98561efbe60..5fb20b15df42535c1ab50a2261fd3e3f73208249 100644
--- a/ui/gfx/color_utils.cc
+++ b/ui/gfx/color_utils.cc
@@ -284,6 +284,11 @@ SkColor AlphaBlend(SkColor foreground, SkColor background, SkAlpha alpha) {
                         static_cast<int>(std::round(b)));
 }
 
+SkColor ImplicitAlphaBlend(SkColor foreground, SkColor background) {
+  return AlphaBlend(SkColorSetA(foreground, SK_AlphaOPAQUE), background,
+                    SkColorGetA(foreground));
+}
+
 bool IsDark(SkColor color) {
   return GetLuma(color) < 128;
 }