Replace TaskType::kUnthrottled with kInternalDefault in FontFaceSet

The task runner is used at HandlePendingEventsAndPromises, and I think
an unthorttled task runner is not necessary here.

Note that the original task runner before kUnthrottled was the default
timer task runner, which has gone. Related CLs:
- https://chromium-review.googlesource.com/c/chromium/src/+/822931
  Use the content's kUnthrottled task runner instead of the default
  timer task runner for FontFaceSet.
- https://chromium-review.googlesource.com/c/chromium/src/+/928186
  Remove the defualt timer task runner.

We plan to eliminate kUnthrottled, kUnspecedTimer and kUnspecedLoading
and replace them with appropriate kInternal* task types.

Task type guideline: https://bit.ly/2vMAsQ4

Bug: 836410
Change-Id: I40fb6bd63ee657b68d86f952d6517254ffbf4b51
Reviewed-on: https://chromium-review.googlesource.com/1055031Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558239}

third_party/blink/renderer/core/css/font_face_set.h

@@ -45,11 +45,10 @@ class CORE_EXPORT FontFaceSet : public EventTargetWithInlineData,
         ready_(new ReadyProperty(GetExecutionContext(),
                                  this,
                                  ReadyProperty::kReady)),
-        // TODO(scheduler-dev): Create an internal task type for fonts.
         async_runner_(AsyncMethodRunner<FontFaceSet>::Create(
             this,
             &FontFaceSet::HandlePendingEventsAndPromises,
-            context.GetTaskRunner(TaskType::kUnthrottled))) {}
+            context.GetTaskRunner(TaskType::kInternalDefault))) {}
   ~FontFaceSet() override = default;
 
   DEFINE_ATTRIBUTE_EVENT_LISTENER(loading);