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Currently performing forward/backward on a tab will reset the TabDownloadState.
Which allows javascript code to do trigger multiple downloads.
This CL disables that behavior by not resetting the TabDownloadState on
forward/back.
It is still possible to reset the TabDownloadState through user gesture
or using browser initiated download.

BUG=848535

Change-Id: I7f9bf6e8fb759b4dcddf5ac0c214e8c6c9f48863
Reviewed-on: https://chromium-review.googlesource.com/1108959
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574437}