[ScopedTaskEnvironment] Clarify that delayed tasks are always QUEUED under MOCK_TIME mode.

Per discussion @
https://groups.google.com/a/chromium.org/d/msg/scheduler-dev/X83HjSx32dI/6cH6L0MsEAAJ

R=carlscab@chromium.org

Bug: None
Change-Id: Id6191dffe2b2572576d4f001be800bc3ac74cc45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716914
Commit-Queue: Gabriel Charette <gab@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#680469}

base/test/scoped_task_environment.h

@@ -133,6 +133,8 @@ class ScopedTaskEnvironment {
     QUEUED,
     // Thread pool tasks run as they are posted. RunUntilIdle() can still be
     // used to block until done.
+    // Note that regardless of this trait, delayed tasks are always "queued"
+    // under TimeSource::MOCK_TIME mode.
     ASYNC,
     DEFAULT = ASYNC
   };