downloads: truncate long search results.

R=estade@chromium.org
BUG=422906

Review URL: https://codereview.chromium.org/651993002

Cr-Commit-Position: refs/heads/master@{#299355}

chrome/browser/resources/search_header.css

@@ -52,4 +52,7 @@ header > :-webkit-any(form, input[type='search']) {
 
 .summary :first-child {
   flex: 1;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
 }