Disables user interaction on UITextView on Sad Tab page.

UITextView defaults to listen to user interaction and editable. This
is not appropriate for sad tab content which should be readonly.

Bug: 738426
Change-Id: I36e131ac37218970c0ee58d7df0b2f0913e93361
Reviewed-on: https://chromium-review.googlesource.com/570384Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486463}

ios/chrome/browser/ui/sad_tab/sad_tab_view.mm

@@ -383,6 +383,7 @@ NSString* const kMessageTextViewBulletRTLFormat = @"\u202E%@\u202C";
                                        alpha:1.0]];
     [_messageTextView setFont:[[MDCTypography fontLoader]
                                   regularFontOfSize:kMessageTextViewFontSize]];
+    [_messageTextView setUserInteractionEnabled:NO];
   }
   return _messageTextView;
 }