[NTP] Keep attribution behind the custom link toast notification

Fix attribution floating above the custom link toast notification.

Bug: 851296
Change-Id: Ibcf9ae6e48f89d6a7fece3fa79455d4004bece05
Reviewed-on: https://chromium-review.googlesource.com/c/1336668
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Ramya Nagarajan <ramyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608482}

chrome/browser/resources/local_ntp/custom_backgrounds.css

@@ -589,6 +589,7 @@ html[dir=rtl] .plus-icon {
   padding: 8px 8px 8px 8px;
   position: fixed;
   text-shadow: 0 0 16px rgba(0, 0, 0, 0.3);
+  z-index: -1;
 }
 
 html[dir=rtl] #custom-bg-attr {

chrome/browser/resources/local_ntp/local_ntp.css

@@ -553,6 +553,18 @@ html[dir=rtl] #mv-notice-x {
   transition: bottom 400ms;
 }
 
+/* Required to allow click events through the wrapping div of the toast
+ * notification container. */
+.md-icons #mv-notice-container,
+#error-notice-container {
+  pointer-events: none;
+}
+.md-icons #mv-notice-container > div,
+#error-notice-container > div {
+  pointer-events: all;
+}
+
+
 .md-icons #mv-notice-container.float-up {
   bottom: 0;
 }