[Blink>Media] Moving autoplay logic to AutoplayPolicy

This CL moves autoplay code from HTMLMediaElement to the
AutoplayPolicy class, which helps decoupling the autoplay logic
from HTMLMediaElement.

BUG=712606

Review-Url: https://codereview.chromium.org/2813303005
Cr-Commit-Position: refs/heads/master@{#466137}

docs/media/autoplay.md

@@ -14,6 +14,13 @@ There are two ways of initiating autoplay:
 * 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

third_party/WebKit/Source/core/html/BUILD.gn

@@ -389,6 +389,8 @@ blink_core_sources("html") {
     "imports/HTMLImportsController.h",
     "imports/LinkImport.cpp",
     "imports/LinkImport.h",
+    "media/AutoplayPolicy.cpp",
+    "media/AutoplayPolicy.h",
     "media/AutoplayUmaHelper.cpp",
     "media/AutoplayUmaHelper.h",
     "media/HTMLMediaElementControlsList.cpp",