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This is first refactoring CL for PassKit implementation based on ios/web
Download API.

Service CL: http://crrev.com/c/801916
UI CL: http://crrev.com/c/794302

Design doc: https://goto.google.com/ios-chrome-passkit

Bug: 787943
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Id5d3f42da40a9ee0da2c8532d646334e6af29917
Reviewed-on: https://chromium-review.googlesource.com/802080Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Gregory Chatzinoff <gchatz@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521792}