Remove uses of GetBlockingPool() from ios/.

This is the ios/ mirror to http://crrev.com/c/575229.

Using NOPRESUBMIT as I move the use of ScopedAllowIO around while
merging two functions and this confuse the presubmit check.

NOPRESUBMIT=true

Bug: 667892
Change-Id: I813b7ea5d3a673ce242644173f870e5faa3977eb
Reviewed-on: https://chromium-review.googlesource.com/618929
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496319}
20 files changed