When editing a Bookmark Folder, Entry. There's a "shield" on the rest of the screen that lets the user know that tapping outside bookmarks won't dismiss it. We should darken that shield

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e2af95f31c04e996aee6f63224a1be02c7e4c7bf

commit e2af95f31c04e996aee6f63224a1be02c7e4c7bf
Author: Chris Lu <thegreenfrog@chromium.org>
Date: Thu Jun 28 00:19:55 2018

[ios] darken TableViewPresentationController modal shield to 0.4 alpha

Before: https://drive.google.com/file/d/1lkH7kzBhoc02yFuqiJFkArO5XGoRkgv4/view?usp=sharing
After: https://drive.google.com/file/d/1HVNhe0k4rWch1oO6TFFLQkU17SL9fqLg/view?usp=sharing

Bug:  856626 
Change-Id: I022b5d8b54828855f836e0428d29d0ffde6bcdfd
Reviewed-on: https://chromium-review.googlesource.com/1117743
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570967}
[modify] https://crrev.com/e2af95f31c04e996aee6f63224a1be02c7e4c7bf/ios/chrome/browser/ui/table_view/table_view_presentation_controller.mm