[ash-md] Changed opaque shelf color darkening from 90% to 70%.

Darkening the shelf by 90% for the --ash-shelf-color experiment was too close to
black. Changing to 70% will make the color more visible.

TBR=sky@chromium.org

BUG=595010

Review-Url: https://codereview.chromium.org/2742833006
Cr-Commit-Position: refs/heads/master@{#456183}

ash/common/shelf/shelf_constants.cc

@@ -23,7 +23,7 @@ const float kShelfInkDropVisibleOpacity = 0.2f;
 const SkColor kShelfIconColor = SK_ColorWHITE;
 const int kShelfTranslucentAlpha = 153;
 const int kShelfTranslucentColorDarkenAlpha = 102;
-const int kShelfOpaqueColorDarkenAlpha = 230;
+const int kShelfOpaqueColorDarkenAlpha = 178;
 const int kOverflowButtonSize = 32;
 const int kOverflowButtonCornerRadius = 2;
 const int kAppListButtonRadius = kOverflowButtonSize / 2;