[NTP] Adjust scrim opacity

Adjust scrim from 20% opacity flat to, 0% (top) – 30% (bottom)

Bug: 867753
Change-Id: I9b99f511c0656b352ca0c2840e6cd829b6d27576
Reviewed-on: https://chromium-review.googlesource.com/1155256
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Commit-Queue: Mathieu Perreault <mathp@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579179}

chrome/browser/resources/local_ntp/custom_backgrounds.js

@@ -110,7 +110,7 @@ customBackgrounds.MENU_ENTRIES = {
 };
 
 customBackgrounds.CUSTOM_BACKGROUND_OVERLAY =
-    'linear-gradient(rgba(0, 0, 0, 0.2), rgba(0, 0, 0, 0.2))';
+    'linear-gradient(rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.3))';
 
 // These shound match the corresponding values in local_ntp.js, that control the
 // mv-notice element.