[Blink>Media] Moving autoplay logic to AutoplayPolicy

docs/media/autoplay.md

 # Autoplay of HTMLMediaElements
 
 Autoplay is the concept of playing media elements without user gesture. On
 desktop, autoplay is always allowed. On mobile, only muted video elements are
 allowed to autoplay. The autoplay logic follows
 the
 [HTML spec](https://html.spec.whatwg.org/multipage/embedded-content.html#media-elements).
 
 There are two ways of initiating autoplay:
 
 * Autoplay by attribute: Setting the `autoplay` attribute on the media element.
   The element will try to autoplay when the `readyState` changes to
   HAVE_ENOUGH_DATA.
 * Autoplay by `play()` method: Explicitly calling the `play()` method without
   user gesture.
 
+All the autoplay logic is handled by the AutoplayPolicy class. When the media
+element wants to perform some action (like unmute, autoplay by attribute or
+`play()` method), it will send a request to AutoplayPolicy, and if the request
+is approved, the element can autoplay, otherwise it should be paused. Also the
+media element should inform the AutoplayPolicy about relevant changes such as
+"the element has been moved to a new document".
+
 ## User gesture lock
 
 Each media element has a user gesture lock. If the element is allowed to
 autoplay, the lock is initialized as `false`, otherwise it's `true`.
 
 When the element is trying to initate autoplay, we check the gesture lock. If
 the lock is `false`, it will be allowed. Otherwise autoplay will be blocked. An
 exception is that if the element is a muted video element, the gesture lock
 check will be bypassed.
 
@@ skipping 13 matching lines @@
 to unmute the video, we check the gesture lock and pause the video if it is
 still `true`.
 
 For autoplay by `play()` method, it is:
 
 ![Autoplay by play() method](autoplay_by_play_method.png)
 
 This means if autoplay is initiated by `play()` method, we continue playing the
 video as normal `play()`. However if the page tries to unmute the video, we check
 the gesture lock and pause the video if it is still `true`.