WebCodecs: never use main thread for audio|video decoding

Prior to this CL, video decoding used the main thread whenever gpu
acceleration is unavailable/disabled, and audio decoding always used the
main thread. Now both cases use a SequencedTaskRunner from the worker
(no relation to web workers) pool.

Also

Bug: 1095786
Change-Id: I1c6265cdb63bcd4dadf48fdbe9bd27622218b948
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500526
Auto-Submit: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823520}

third_party/blink/renderer/modules/webcodecs/audio_decoder_broker.cc

@@ -26,6 +26,7 @@
 #include "third_party/blink/renderer/modules/webcodecs/decoder_selector.h"
 #include "third_party/blink/renderer/platform/scheduler/public/post_cross_thread_task.h"
 #include "third_party/blink/renderer/platform/scheduler/public/thread.h"
+#include "third_party/blink/renderer/platform/scheduler/public/worker_pool.h"
 #include "third_party/blink/renderer/platform/wtf/cross_thread_functional.h"
 #include "third_party/blink/renderer/platform/wtf/functional.h"
 
@@ -259,10 +260,9 @@ class MediaAudioTaskWrapper {
 constexpr char AudioDecoderBroker::kDefaultDisplayName[];
 
 AudioDecoderBroker::AudioDecoderBroker(ExecutionContext& execution_context)
-    : media_task_runner_(
-          // TODO(chcunningham): This should use a separate thread from the
-          // pool. http://crbug.com/1095786
-          execution_context.GetTaskRunner(TaskType::kInternalMedia)) {
+    // Use a worker task runner to avoid scheduling decoder
+    // work on the main thread.
+    : media_task_runner_(worker_pool::CreateSequencedTaskRunner({})) {
   DVLOG(2) << __func__;
   media_tasks_ = std::make_unique<MediaAudioTaskWrapper>(
       weak_factory_.GetWeakPtr(), execution_context, media_task_runner_,

third_party/blink/renderer/modules/webcodecs/video_decoder_broker.cc

@@ -30,6 +30,7 @@
 #include "third_party/blink/renderer/modules/webcodecs/decoder_selector.h"
 #include "third_party/blink/renderer/platform/scheduler/public/post_cross_thread_task.h"
 #include "third_party/blink/renderer/platform/scheduler/public/thread.h"
+#include "third_party/blink/renderer/platform/scheduler/public/worker_pool.h"
 #include "third_party/blink/renderer/platform/wtf/cross_thread_functional.h"
 #include "third_party/blink/renderer/platform/wtf/functional.h"
 #include "ui/gfx/color_space.h"
@@ -299,14 +300,11 @@ VideoDecoderBroker::VideoDecoderBroker(
     media::GpuVideoAcceleratorFactories* gpu_factories)
     : media_task_runner_(
           gpu_factories
+              // GpuFactories requires we use its task runner when available.
               ? gpu_factories->GetTaskRunner()
-              // TODO(chcunningham): Consider adding a new single thread task
-              // runner just for WebCodecs. This is still using the main thread,
-              // albeit at a lower priority than things like user gestures.
-              // http://crbug.com/1095786
-              // TODO(chcunningham): Should this be kInternalMediaRealTime? Why
-              // does WebAudio use that task type?
-              : execution_context.GetTaskRunner(TaskType::kInternalMedia)) {
+              // Otherwise, use a worker task runner to avoid scheduling decoder
+              // work on the main thread.
+              : worker_pool::CreateSequencedTaskRunner({})) {
   DVLOG(2) << __func__;
   media_tasks_ = std::make_unique<MediaVideoTaskWrapper>(
       weak_factory_.GetWeakPtr(), execution_context, gpu_factories,

third_party/blink/renderer/platform/scheduler/common/worker_pool.cc

@@ -23,6 +23,11 @@ void PostTask(const base::Location& location,
                              ConvertToBaseOnceCallback(std::move(closure)));
 }
 
+scoped_refptr<base::SequencedTaskRunner> CreateSequencedTaskRunner(
+    const base::TaskTraits& traits) {
+  return base::ThreadPool::CreateSequencedTaskRunner(traits);
+}
+
 }  // namespace worker_pool
 
 }  // namespace blink

third_party/blink/renderer/platform/scheduler/public/worker_pool.h

@@ -6,6 +6,7 @@
 #define THIRD_PARTY_BLINK_RENDERER_PLATFORM_SCHEDULER_PUBLIC_WORKER_POOL_H_
 
 #include "base/location.h"
+#include "base/memory/scoped_refptr.h"
 #include "base/sequenced_task_runner.h"
 #include "base/task/task_traits.h"
 #include "third_party/blink/renderer/platform/platform_export.h"
@@ -31,8 +32,8 @@ PLATFORM_EXPORT void PostTask(const base::Location&,
                               const base::TaskTraits&,
                               CrossThreadOnceClosure);
 
-// TODO(altimin): Expose CreateSequencedTaskRunner when the
-// need arises.
+PLATFORM_EXPORT scoped_refptr<base::SequencedTaskRunner>
+CreateSequencedTaskRunner(const base::TaskTraits& traits);
 
 }  // namespace worker_pool