MD downloads: use [placeholder] instead of <label>

chrome/browser/resources/md_downloads/toolbar.css

 /* Copyright 2015 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 :host {
   @apply(--downloads-shared-style);
   align-items: center;
   background: rgb(63, 85, 102);
   color: white;
   content-sizing: padding-box;
@@ skipping 27 matching lines @@
 }
 
 #search-term {
   --paper-input-container-color: rgba(255, 255, 255, .5);
   --paper-input-container-focus-color: white;
   --paper-input-container-input: {
     font-family: inherit;
     font-size: 14px;
   };
   --paper-input-container-input-color: white;
-  --paper-input-container-label: {
-    font-family: inherit;
-    font-size: 14px;
-  };
   -webkit-margin-end: 8px;
   z-index: 0;
 }
 
 #search-term input[type='search']::-webkit-search-cancel-button,
 #search-term input[type='search']::-webkit-search-results-button {
   -webkit-appearance: none;
 }
 
 #search-term paper-icon-button {
@@ skipping 67 matching lines @@
     display: none;
   }
 }
 
 @media all and (max-width: 1024px) {
   /* Hide #actions for narrow windows; they're shown in a vertical dot menu. */
   #actions {
     display: none;
   }
 }