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It was never used and never will be as the IO thread is being
phased out altogether.

This is a partial revert of https://chromium-review.googlesource.com/c/chromium/src/+/1660340

This CL keeps (diff with the raw revert on PS1):
 1) TaskQueueThrottler's Metadata inner class which isolates some logic
 2) TaskObserver::OnQueueNextWakeUpChanged dropping the unused TaskQueue* param
 3) TaskQueueImpl::SetObserver in favor of TaskQueueImpl::SetOnNextWakeUpChangedCallback
   (more adaptable to future instrumentation)
 4) BrowserTaskQueues::Handle as a ref-counted class
   (could go back to being copied around but avoiding duplicate state
    makes state management easier and even essential in some cases like
    validator_sets_ used to require)
   Note: The provider of Handle at the time has been renamed from
         BrowserIOTaskEnvironment to BrowserIOThreadDelegate if you're
         looking for it.
   Side-effect: this keeps all the . to -> usage changes for Handle.

And removes:
 A) TaskQueueObserver::OnPostTask
 B) BrowserTaskQueues::Validator
 C) ScopedDoNotUseUIDefaultQueueFromIO

This is associated to crbug.com/1026641 because supporting
BrowserTaskQueues::Validator adds an unnecessary burden (not that it'd
be impossible to re-add them if explicitly necessary post-migration).

TBR=nasko@chromium.org (BUILD.gn)

Bug: 863341, 1026641
Change-Id: I7bfdc8730cacbac33e2e9b57d1936b6755033753
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2008462
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734660}