Make showcase home screen not translucent.

EarlGrey 1.10.1 has a bug that the scrollToTop action hangs
indefinitely if the UITableView's navigation bar is translucent. Given
that it makes no difference whether the view is translucent or not,
this CL makes it not translucent to unblock rolling EarlGrey to 1.10.1.

Bug: 747116
Change-Id: Idd48cf6f3f024c4b88bedcca412692598cd1a493
Reviewed-on: https://chromium-review.googlesource.com/580357Reviewed-by: Ed Chin <edchin@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488417}

ios/showcase/core/showcase_view_controller.mm

@@ -68,6 +68,7 @@ NSString* const kUseCaseKey = @"useCase";
   self.searchController.searchResultsUpdater = self;
   self.searchController.dimsBackgroundDuringPresentation = NO;
   self.tableView.tableHeaderView = self.searchController.searchBar;
+  self.navigationController.navigationBar.translucent = NO;
 
   // Presentation of searchController will walk up the view controller hierarchy
   // until it finds the root view controller or one that defines a presentation

ios/showcase/tools_menu/sc_tools_coordinator.mm

@@ -53,10 +53,6 @@
   // The Overflow controls will only be displayed on CompactWidth SizeClasses.
   [viewController setDisplayOverflowControls:YES];
 
-  // Since the close MenuButton is always located on the top right corner,
-  // set the navigation translucency to NO so it doesn't cover the button.
-  self.baseViewController.navigationBar.translucent = NO;
-
   [self.baseViewController pushViewController:viewController animated:YES];
 }