Retire FeaturePolicyAutoplayFeature flag.

Shipped in M64, this feature allows developers to selectively enable
and disable use of autoplay through the feature policy HTTP header
or the <iframe> "allow" attribute.

https://chromium-review.googlesource.com/831520
https://www.chromestatus.com/features/5100524789563392

BUG=788390

Change-Id: I0f4346bc75dc922c8004f0008921d68a5a3beede
Reviewed-on: https://chromium-review.googlesource.com/1166614
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581968}

third_party/blink/renderer/core/html/media/autoplay_policy.cc

@@ -113,8 +113,7 @@ bool AutoplayPolicy::IsDocumentAllowedToPlay(const Document& document) {
       return true;
     }
 
-    if (!RuntimeEnabledFeatures::FeaturePolicyAutoplayFeatureEnabled() ||
-        !frame->IsFeatureEnabled(mojom::FeaturePolicyFeature::kAutoplay)) {
+    if (!frame->IsFeatureEnabled(mojom::FeaturePolicyFeature::kAutoplay)) {
       return false;
     }
   }

third_party/blink/renderer/platform/feature_policy/feature_policy.cc

@@ -210,6 +210,8 @@ void AllowFeatureEverywhere(mojom::FeaturePolicyFeature feature,
 const FeatureNameMap& GetDefaultFeatureNameMap() {
   DEFINE_STATIC_LOCAL(FeatureNameMap, default_feature_name_map, ());
   if (default_feature_name_map.IsEmpty()) {
+    default_feature_name_map.Set("autoplay",
+                                 mojom::FeaturePolicyFeature::kAutoplay);
     default_feature_name_map.Set("camera",
                                  mojom::FeaturePolicyFeature::kCamera);
     default_feature_name_map.Set("encrypted-media",
@@ -249,10 +251,6 @@ const FeatureNameMap& GetDefaultFeatureNameMap() {
       default_feature_name_map.Set("sync-script",
                                    mojom::FeaturePolicyFeature::kSyncScript);
     }
-    if (RuntimeEnabledFeatures::FeaturePolicyAutoplayFeatureEnabled()) {
-      default_feature_name_map.Set("autoplay",
-                                   mojom::FeaturePolicyFeature::kAutoplay);
-    }
     if (RuntimeEnabledFeatures::PaymentRequestEnabled()) {
       default_feature_name_map.Set("payment",
                                    mojom::FeaturePolicyFeature::kPayment);

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -455,10 +455,6 @@
     {
       name: "FastMobileScrolling",
     },
-    {
-      name: "FeaturePolicyAutoplayFeature",
-      status: "stable"
-    },
     {
       name: "FeaturePolicyJavaScriptInterface",
       origin_trial_feature_name: "FeaturePolicyJSAPI",