[Chromecast] Enable main frame autoplay delegation

This forces autoplay on for all main frames, and turns on the autoplay
delegation mechanism via <iframe allow="autoplay" />. This turns off the
previous behavior of unconditionally allowing autoplay on all pages and
frames.

Bug: internal b/160651728
Test: Launch AMP Viewer via web sender, verify only the screen reader is
      active while videos/ads remain paused.

Change-Id: If570ca3911596bf27bcceb56360fad674a9236c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2296360Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789315}

chromecast/browser/DEPS

@@ -61,6 +61,7 @@ include_rules = [
   "+services/service_manager/embedder",
   "+storage/browser/quota/quota_settings.h",
   "+third_party/blink/public/common",
+  "+third_party/blink/public/mojom/autoplay",
   "+third_party/blink/public/mojom/loader/resource_load_info.mojom.h",
   "+third_party/blink/public/mojom/mediastream/media_stream.mojom-shared.h",
   "+third_party/blink/public/mojom/messaging",

chromecast/browser/cast_browser_main_parts.cc

@@ -345,9 +345,6 @@ const DefaultCommandLineSwitch kDefaultSwitches[] = {
     {switches::kEnableUseZoomForDSF, "false"},
     // TODO(halliwell): Revert after fix for b/63101386.
     {switches::kDisallowNonExactResourceReuse, ""},
-    // Enable autoplay without requiring any user gesture.
-    {switches::kAutoplayPolicy,
-     switches::autoplay::kNoUserGestureRequiredPolicy},
     // Disable pinch zoom gesture.
     {switches::kDisablePinch, ""},
 };

chromecast/browser/cast_web_contents_impl.cc

@@ -41,6 +41,7 @@
 #include "net/base/net_errors.h"
 #include "services/service_manager/public/cpp/interface_provider.h"
 #include "third_party/blink/public/common/associated_interfaces/associated_interface_provider.h"
+#include "third_party/blink/public/mojom/autoplay/autoplay.mojom.h"
 #include "third_party/blink/public/mojom/favicon/favicon_url.mojom.h"
 #include "third_party/blink/public/mojom/loader/resource_load_info.mojom.h"
 #include "third_party/skia/include/core/SkColor.h"
@@ -629,6 +630,26 @@ void CastWebContentsImpl::ReadyToCommitNavigation(
   DCHECK(navigation_handle);
   if (!web_contents_ || closing_ || stopped_)
     return;
+
+  // We want to honor the autoplay feature policy (via allow="autoplay") without
+  // explicit user activation, since media on Cast is extremely likely to have
+  // already been explicitly requested by a user via voice or over the network.
+  // By spoofing the "high media engagement" signal, we can bypass the user
+  // gesture requirement for autoplay.
+  int32_t autoplay_flags = blink::mojom::kAutoplayFlagHighMediaEngagement;
+
+  // Main frames should have autoplay enabled by default, since autoplay
+  // delegation via parent frame doesn't work here.
+  if (navigation_handle->IsInMainFrame())
+    autoplay_flags |= blink::mojom::kAutoplayFlagForceAllow;
+
+  mojo::AssociatedRemote<blink::mojom::AutoplayConfigurationClient> client;
+  navigation_handle->GetRenderFrameHost()
+      ->GetRemoteAssociatedInterfaces()
+      ->GetInterface(&client);
+  auto autoplay_origin = url::Origin::Create(navigation_handle->GetURL());
+  client->AddAutoplayFlags(autoplay_origin, autoplay_flags);
+
   if (!navigation_handle->IsInMainFrame())
     return;