[blink] Decrement scheduler idle tasks before running an idle task.

To maintain the invariant that
  `num_scheduler_idle_tasks_` <= `idle_tasks_.size()`
after invoking `RemoveCancelledIdleTasks()` on the scheduler, even if
the idle task cancels itself (without this, an idle task that cancels
itself would be counted in `num_scheduler_idle_tasks_` while running,
but not in  `idle_tasks_.size()`.

Bug: 394266102, 365114039
Change-Id: Ib9b6502bdd9ca99b0433c0fb03313f190d4f4a57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6277467
Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Auto-Submit: Francois Pierre Doray <fdoray@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1421552}