Add 'autoplay' content settings and expose it to Blink.

chrome/browser/chrome_content_browser_client.cc

Index: chrome/browser/chrome_content_browser_client.cc
diff --git a/chrome/browser/chrome_content_browser_client.cc b/chrome/browser/chrome_content_browser_client.cc
index 2a8495aa37f96841351f0e02190f21ef21e084de..7122f104469d21db687286a7a8b6d32e1543d66c 100644
--- a/chrome/browser/chrome_content_browser_client.cc
+++ b/chrome/browser/chrome_content_browser_client.cc
@@ -673,6 +673,12 @@ void GetGuestViewDefaultContentSettingRules(
                                   CONTENT_SETTING_ALLOW,
                                   std::string(),
                                   incognito));
+  rules->autoplay_rules.push_back(
+      ContentSettingPatternSource(ContentSettingsPattern::Wildcard(),
+                                  ContentSettingsPattern::Wildcard(),
+                                  CONTENT_SETTING_ALLOW,
+                                  std::string(),
+                                  incognito));
 }
 #endif  // defined(ENABLE_EXTENSIONS)