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

chrome/browser/flag_descriptions.cc

Index: chrome/browser/flag_descriptions.cc
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index 381a31eed59ab9dfa752bdd56e58ed8d376de4bd..55483c4294fe7bddc3385ba100461b0af17a5244 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -3114,6 +3114,9 @@ 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";