CrOS Shelf: add background over app list

Currently the shelf is fully transparent when the app
list is open. In thr new UI, we want to make it slightly
darker to distinguish it from the app list.

Bug: 878601
Change-Id: I5167e684e174c2d48334f45fa9b1c66e79ba32b7
Reviewed-on: https://chromium-review.googlesource.com/1194067Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587137}

ash/shelf/shelf_background_animator.cc

@@ -84,7 +84,11 @@ std::pair<int, int> GetTargetColorAlphaValues(
       target_item_color_alpha = SK_AlphaTRANSPARENT;
       break;
     case SHELF_BACKGROUND_APP_LIST:
-      target_shelf_color_alpha = SK_AlphaTRANSPARENT;
+      if (chromeos::switches::ShouldUseShelfNewUi()) {
+        target_shelf_color_alpha = kShelfTranslucentOverAppList;
+      } else {
+        target_shelf_color_alpha = SK_AlphaTRANSPARENT;
+      }
       target_item_color_alpha = SK_AlphaTRANSPARENT;
       break;
     case SHELF_BACKGROUND_SPLIT_VIEW:

ash/shelf/shelf_constants.h

@@ -65,6 +65,7 @@ constexpr float kShelfInkDropVisibleOpacity = 0.2f;
 ASH_EXPORT constexpr SkColor kShelfIconColor = SK_ColorWHITE;
 
 // The alpha value for the shelf background.
+ASH_EXPORT constexpr int kShelfTranslucentOverAppList = 77;            // 30%
 ASH_EXPORT constexpr int kShelfTranslucentAlpha = 153;                 // 60%
 ASH_EXPORT constexpr int kShelfTranslucentMaximizedWindowNewUi = 230;  // 90%