CrOS shelf: fix background alpha test

This is probably some weird condition between submitting
change 1194067 (change shelf background over app list) and
change 1188848 (enable new shelf UI by default).

Bug: 878601
Change-Id: I0ffb0ef3e27f9b4412a7e75409d70c367e53326e
Reviewed-on: https://chromium-review.googlesource.com/1194509Reviewed-by: Vladislav Kaznacheev <kaznacheev@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587348}

ash/shelf/shelf_background_animator_unittest.cc

@@ -272,7 +272,7 @@ TEST_F(ShelfBackgroundAnimatorTest, MAYBE_FullscreenAppListBackground) {
   PaintBackground(SHELF_BACKGROUND_APP_LIST);
 
   EXPECT_EQ(SHELF_BACKGROUND_APP_LIST, animator_->target_background_type());
-  EXPECT_EQ(0, observer_.GetBackgroundAlpha());
+  EXPECT_EQ(kShelfTranslucentOverAppList, observer_.GetBackgroundAlpha());
   EXPECT_EQ(0, observer_.GetItemBackgroundAlpha());
 }