CrOS Shelf: Remove bg color from tray items when 'in-app'

See the spec at https://guidelines.googleplex.com/crosux-spec/feature-specs/shelf-tablet.html#shelf-tablet-home
("in-app shelf").

Bug: 982512
Change-Id: Ie152168d734457ba735b73b4c1514c975ea15a4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824358
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699588}

ash/system/tray/tray_background_view.cc

@@ -129,8 +129,14 @@ class TrayBackground : public views::Background {
     cc::PaintFlags background_flags;
     background_flags.setAntiAlias(true);
     int border_radius = kTrayRoundedBorderRadius;
+    const bool tablet_mode =
+        Shell::Get()->tablet_mode_controller() &&
+        Shell::Get()->tablet_mode_controller()->InTabletMode();
     background_flags.setColor(
-        ShelfConfig::Get()->shelf_control_permanent_highlight_background());
+        tablet_mode && ShelfConfig::Get()->is_in_app()
+            ? SK_ColorTRANSPARENT
+            : ShelfConfig::Get()
+                  ->shelf_control_permanent_highlight_background());
     border_radius = ShelfConfig::Get()->control_border_radius();
 
     gfx::Rect bounds = tray_background_view_->GetBackgroundBounds();