DevTools: Don't show blank placeholder in TextPrompt

third_party/WebKit/Source/devtools/front_end/ui/textPrompt.css

 /*
  * Copyright (c) 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.
  */
 
 .text-prompt-editing {
     -webkit-user-select: text;
     box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.2),
                 0 2px 4px rgba(0, 0, 0, 0.2),
@@ skipping 17 matching lines @@
     -webkit-user-modify: read-write-plaintext-only;
     white-space: pre;
 }
 
 ::content .auto-complete-text {
     color: rgb(128, 128, 128) !important;
     -webkit-user-select: none;
     -webkit-user-modify: read-only;
 }
 
-::content .text-prompt:empty::before {
+::content .text-prompt[data-placeholder]:empty::before {
     content: attr(data-placeholder);
     color: rgb(128, 128, 128);
 }
 
 .text-prompt-editing ::content br {
     display: none;
 }