The current PaymentInstrumentIconFetcher class is too complicated and
can not ensure thread-safety due to misuse some APIs. So, this patch
refactors the class to use ManifestIconDownloader internally. After
this patch, we can remove much duplicated codes and resolve some
crashes in debug mode as well.

Bug: 752836
Change-Id: I04c01d433d093ae138148e6aa2849bc1713c1c85
Reviewed-on: https://chromium-review.googlesource.com/658201
Commit-Queue: Jinho Bang <jinho.bang@samsung.com>
Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501589}