Add placeholder text for url bar in //ios/web_view shell app.

It says "Search or type URL" like in //ios/chrome.

Change-Id: I3d58b0487f76e174e9779f361c28869833fe943e
Reviewed-on: https://chromium-review.googlesource.com/c/1448118
Auto-Submit: John Wu <jzw@chromium.org>
Reviewed-by: Hiroshi Ichikawa <ichikawa@chromium.org>
Commit-Queue: Hiroshi Ichikawa <ichikawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628184}

ios/web_view/shell/shell_view_controller.m

@@ -141,6 +141,7 @@ NSString* const kWebViewShellJavaScriptDialogTextFieldAccessibiltyIdentifier =
                   action:@selector(showMenu)
         forControlEvents:UIControlEventTouchUpInside];
 
+  _field.placeholder = @"Search or type URL";
   _field.backgroundColor = [UIColor whiteColor];
   _field.tintColor = _headerBackgroundView.backgroundColor;
   [_field setContentHuggingPriority:UILayoutPriorityDefaultLow - 1