Remove WebThread::GetBlockingPool usage for IOSImageDecoderImpl

Removes the plumbing for the pool and creates it directly in the class.

Bug: 667892
Change-Id: Idf70d171f20d5f649978ef36db9988c4918430f5
Reviewed-on: https://chromium-review.googlesource.com/591128
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490752}
7 files changed