Add login shelf background color constant.

Fix bug where login shelf buttons background color constant and darken
wallpaper background color constant were combined incorrectly.

Bug: 1137798
Change-Id: Ib0584a990bcb40c4886ede2e402d2f8864705597
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487831Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Min Chen <minch@chromium.org>
Reviewed-by: Yulun Wu <yulunwu@chromium.org>
Commit-Queue: Yulun Wu <yulunwu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819153}

ash/shelf/login_shelf_view.cc

@@ -93,7 +93,7 @@ SkColor GetButtonBackgroundColor() {
       session_manager::SessionState::OOBE) {
     return SkColorSetA(SK_ColorBLACK, 16);  // 6% opacity
   }
-  return DeprecatedGetLoginBackgroundBaseColor(kLoginBackgroundBaseColor);
+  return DeprecatedGetLoginBackgroundBaseColor(kLoginButtonBackgroundBaseColor);
 }
 
 LoginMetricsRecorder::ShelfButtonClickTarget GetUserClickTarget(int button_id) {

ash/style/default_color_constants.h

@@ -19,6 +19,8 @@ constexpr SkColor kPowerButtonMenuFullscreenShieldColor = SK_ColorBLACK;
 constexpr SkColor kLoginShelfButtonLabelColor = gfx::kGoogleGrey100;
 constexpr SkColor kLoginShelfButtonIconColor = SkColorSetRGB(0xEB, 0xEA, 0xED);
 constexpr SkColor kLoginBackgroundBaseColor = SK_ColorBLACK;
+constexpr SkColor kLoginButtonBackgroundBaseColor =
+    SkColorSetA(SK_ColorWHITE, 26);
 
 // Colors for shelf.
 constexpr SkColor kDefaultShelfInkDropColor = SK_ColorBLACK;