Animation worklet to always use compositor task queue

Split off from https://crrev.com/c/1751324

We want to prioritize loading and compositing task queues during early
load. For correct functioning of web tests we also need to prioritize
main thread animation worklet tasks. In general it makes sense for all
animation tasks to be posted to a compositor task queue regardless of
which thread they are on.

Bug: 971191
Change-Id: I9d26e1d8a54a3bb24a96781968890c277f795011
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762244Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689375}

third_party/blink/renderer/platform/graphics/animation_worklet_mutator_dispatcher_impl.cc

@@ -17,6 +17,7 @@
 #include "third_party/blink/renderer/platform/instrumentation/tracing/trace_event.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/thread_scheduler.h"
 #include "third_party/blink/renderer/platform/wtf/cross_thread_functional.h"
 
 namespace blink {
@@ -68,9 +69,12 @@ AnimationWorkletMutatorDispatcherImpl::AnimationWorkletMutatorDispatcherImpl(
     bool main_thread_task_runner)
     : client_(nullptr), outputs_(OutputVectorRef::Create()) {
   // By default web tests run without threaded compositing. See
-  // https://crbug.com/770028 For these situations we run on the Main thread.
+  // https://crbug.com/770028. If threaded compositing is disabled or
+  // |main_thread_task_runner| is true we run on the main thread's compositor
+  // task runner otherwise we run tasks on the compositor thread's default
+  // task runner.
   host_queue_ = main_thread_task_runner || !Thread::CompositorThread()
-                    ? Thread::MainThread()->GetTaskRunner()
+                    ? Thread::MainThread()->Scheduler()->CompositorTaskRunner()
                     : Thread::CompositorThread()->GetTaskRunner();
   tick_clock_ = std::make_unique<base::DefaultTickClock>();
 }