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Response's MIME type can change during the download and DownloadTask
should support the change. This will allow to report
DOWNLOAD_PASS_KIT_WRONG_MIME_TYPE_FAILURE metric for PassKit downloads.

Bug: 787943
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I137f9aa4ff966c140a9a6cb5b03dc026062bc0b7
Reviewed-on: https://chromium-review.googlesource.com/786265Reviewed-by: Gregory Chatzinoff <gchatz@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519521}