Revert of Use TaskScheduler instead of WorkerPool in v8_platform.cc. (patchset #18 id:340001 of https://codereview.chromium.org/2610473002/ )

Reason for revert:
Causing extensions_unittests to hang and take 15 minutes.

BUG=694828

Original issue's description:
> Use TaskScheduler instead of WorkerPool in v8_platform.cc.
>
> The following traits are used:
>
> Priority: Inherited (default)
>   The priority is inherited from the calling context (i.e. TaskTraits
>   are initialized with the priority of the current task).
>
> Shutdown behavior: CONTINUE_ON_SHUTDOWN
>   Tasks posted with this mode which have not started executing before
>   shutdown is initiated will never run. Tasks with this mode running at
>   shutdown will be ignored (the worker will not be joined).
>
>   Note: Tasks that were previously posted to base::WorkerPool should
>   use this shutdown behavior because this is how base::WorkerPool
>   handles all its tasks.
>
> MayBlock():
>   The task may block.
>
> BUG=659191
>
> Review-Url: https://codereview.chromium.org/2610473002
> Cr-Commit-Position: refs/heads/master@{#449976}
> Committed: https://chromium.googlesource.com/chromium/src/+/b83be4ca0e00b50b17619adf7f7de275455e9852

TBR=jochen@chromium.org,fdoray@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=659191

Review-Url: https://codereview.chromium.org/2711703002
Cr-Commit-Position: refs/heads/master@{#452058}