cocoa: use window background for collected cookies

This dialog looks odd with the default white background of a constrained
window.

Bug: 743120
Change-Id: Id9870b246e8d46310870adef2a95ca4f5f4bd0a1
Reviewed-on: https://chromium-review.googlesource.com/746525Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512824}

chrome/browser/ui/cocoa/content_settings/collected_cookies_mac.mm

@@ -141,6 +141,8 @@ void CollectedCookiesMac::OnConstrainedWindowClosed(
   NSImage* infoIcon = rb.GetNativeImageNamed(IDR_INFO).ToNSImage();
   [infoBarIcon_ setImage:infoIcon];
 
+  [self.window setBackgroundColor:[NSColor windowBackgroundColor]];
+
   // Initialize the banner gradient and stroke color.
   NSColor* bannerStartingColor =
       [NSColor colorWithCalibratedRed:kBannerGradientColorTop[0]