Revert "Make showcase home screen not translucent."

This reverts commit 17aced14.

Reason for revert: I only tested on iPhone 7, however, it seems that testNavigationByTappingReturn is broken on iPhone 5s. I'm going to investigate and then reland. Sorry for the inconvenience.

Original change's description:
> 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/580357
> Reviewed-by: Ed Chin <edchin@chromium.org>
> Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#488417}

TBR=edchin@chromium.org,liaoyuke@chromium.org

Change-Id: I2669a23928d1f4ce230bb25bd852a6f441019f17
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 747116
Reviewed-on: https://chromium-review.googlesource.com/580194Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488446}

ios/showcase/core/showcase_view_controller.mm

@@ -68,7 +68,6 @@ 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

@@ -53,6 +53,10 @@
   // 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];
 }