MD Downloads: add "waterfall" shadow (shows when the list is scrolled)

chrome/browser/resources/md_downloads/vulcanized.html

Index: chrome/browser/resources/md_downloads/vulcanized.html
diff --git a/chrome/browser/resources/md_downloads/vulcanized.html b/chrome/browser/resources/md_downloads/vulcanized.html
index 5bf72194c3cf3f84257963b2f77025f6be931e18..598a499d5e97d80c033141fc2b83bb5aaa8bb4eb 100644
--- a/chrome/browser/resources/md_downloads/vulcanized.html
+++ b/chrome/browser/resources/md_downloads/vulcanized.html
@@ -3469,6 +3469,7 @@ paper-button {
   flex: 1 0;
   flex-direction: column;
   height: 100%;
+  z-index: 0;
 }
 
 @media screen and (max-width: 1024px) {
@@ -3478,6 +3479,27 @@ paper-button {
 
 }
 
+#toolbar {
+  position: relative;
+  z-index: 1;
+}
+
+#toolbar::after {
+  box-shadow: inset 0 5px 6px -3px rgba(0, 0, 0, 0.4);
+  content: '';
+  display: block;
+  height: 6px;
+  opacity: 0;
+  position: absolute;
+  top: 100%;
+  transition: opacity 500ms;
+  width: 100%;
+}
+
+:host([has-shadow_]) #toolbar::after {
+  opacity: 1;
+}
+
 #downloads-list {
   overflow-y: overlay !important;
 }