Autoplay: add document user activation flag in chrome://flags

chrome/browser/flag_descriptions.cc

 // Copyright 2017 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.
 
 #include "chrome/browser/flag_descriptions.h"
 
 namespace flag_descriptions {
 
 //  Material Design version of chrome://bookmarks
 
@@ skipping 3096 matching lines @@
     "Policy used when deciding if audio or video is allowed to autoplay.";
 
 const char kAutoplayPolicyNoUserGestureRequired[] =
     "No user gesture is required.";
 
 const char kAutoplayPolicyUserGestureRequired[] = "User gesture is required.";
 
 const char kAutoplayPolicyUserGestureRequiredForCrossOrigin[] =
     "User gesture is required for cross-origin iframes.";
 
+const char kAutoplayPolicyDocumentUserActivation[] =
+    "Document user activation is required.";
+
 const char kOmniboxDisplayTitleForCurrentUrlName[] =
     "Include title for the current URL in the omnibox";
 
 const char kOmniboxDisplayTitleForCurrentUrlDescription[] =
     "In the event that the omnibox provides suggestions on-focus, the URL of "
     "the current page is provided as the first suggestion without a title. "
     "Enabling this flag causes the title to be displayed.";
 
 const char kOmniboxUIMaxAutocompleteMatchesName[] =
     "Omnibox UI Max Autocomplete Matches";
@@ skipping 101 matching lines @@
 
 const char kDisableNewVirtualKeyboardBehaviorName[] =
     "New window behavior for the accessibility keyboard";
 const char kDisableNewVirtualKeyboardBehaviorDescription[] =
     "Disable new window behavior for the accessibility keyboard "
     "in non-sticky mode (do not change work area in non-sticky mode).";
 
 #endif  // defined(OS_CHROMEOS)
 
 }  // namespace flag_descriptions