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

media/base/media_switches.cc

Index: media/base/media_switches.cc
diff --git a/media/base/media_switches.cc b/media/base/media_switches.cc
index aad4a7ea83fb7d07b3114fdb14e09fb1906efc10..af8c99552ddf9b876a91e3aecd909fc7050e695b 100644
--- a/media/base/media_switches.cc
+++ b/media/base/media_switches.cc
@@ -164,6 +164,10 @@ const char kEnableInternalMediaSession[] = "enable-internal-media-session";
 
 namespace autoplay {
 
+// Autoplay policy that requires a document user activation.
+const char kDocumentUserActivationRequiredPolicy[] =
+    "document-user-activation-required";
+
 // Autoplay policy that does not require any user gesture.
 const char kNoUserGestureRequiredPolicy[] = "no-user-gesture-required";