Use NORMAL priority thread for OfflineAudioWorkletThread

The BACKGROUND thread priority impl on Windows 7/10 is confusing [2],
and it seems very flaky [1]. The team agreed that using the NORMAL
priority thread for OfflineAudioWorkletThread is a sensible fix
for this.

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=1124980#c7
[2] https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyVgsSBUZsYWtlIktjaHJvbWl1bUBibGlua191bml0dGVzdHNAQXVkaW9Xb3JrbGV0VGhyZWFkUHJpb3JpdHlUZXN0LkNoZWNrVGhyZWFkUHJpb3JpdHkM

Bug: 1124980
Change-Id: I8b4063f9a6ee802cafd4a05daaec23bf5f0170d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2411395Reviewed-by: Raymond Toy <rtoy@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807518}

third_party/blink/renderer/modules/webaudio/audio_worklet_thread_test.cc

@@ -318,12 +318,12 @@ constexpr ThreadPriorityTestParam kThreadPriorityTestParams[] = {
     // for a real-time context.
     {true, false, true, base::ThreadPriority::REALTIME_AUDIO},
 
-    // OfflineAudioWorkletThread is always a BACKGROUND priority no matter what
+    // OfflineAudioWorkletThread is always a NORMAL priority no matter what
     // the flag setting or the originating frame level is.
-    {false, true, true, base::ThreadPriority::BACKGROUND},
-    {false, true, false, base::ThreadPriority::BACKGROUND},
-    {false, false, true, base::ThreadPriority::BACKGROUND},
-    {false, false, false, base::ThreadPriority::BACKGROUND},
+    {false, true, true, base::ThreadPriority::NORMAL},
+    {false, true, false, base::ThreadPriority::NORMAL},
+    {false, false, true, base::ThreadPriority::NORMAL},
+    {false, false, false, base::ThreadPriority::NORMAL},
 };
 
 class AudioWorkletThreadPriorityTest

third_party/blink/renderer/modules/webaudio/offline_audio_worklet_thread.cc

@@ -24,8 +24,8 @@ OfflineAudioWorkletThread::OfflineAudioWorkletThread(
   ThreadCreationParams params =
       ThreadCreationParams(ThreadType::kOfflineAudioWorkletThread);
 
-  // OfflineAudioWorkletThread always uses a BACKGROUND priority thread.
-  params.thread_priority = base::ThreadPriority::BACKGROUND;
+  // OfflineAudioWorkletThread always uses a NORMAL priority thread.
+  params.thread_priority = base::ThreadPriority::NORMAL;
 
   if (++s_ref_count_ == 1)
     EnsureSharedBackingThread(params);

third_party/blink/renderer/modules/webaudio/offline_audio_worklet_thread.h

@@ -16,7 +16,7 @@ struct ThreadCreationParams;
 
 // OfflineAudioWorkletThread is a per-AudioWorkletGlobalScope object that has a
 // reference count to the backing thread that performs AudioWorklet tasks.
-// Its backing thread uses BACKGROUND priority no matter what the environment
+// Its backing thread uses NORMAL priority no matter what the environment
 // or the feature flag setting is.
 class MODULES_EXPORT OfflineAudioWorkletThread final : public WorkerThread {
  public: