Change post request to get in didReceiveServerRedirectForProvisionalNavigation.

This was copied from UpdatePendingItemUrl method which will be removed
in a separate CL.

Also reset _lastTransferTimeInSeconds for server redirects.

Bug: 805900
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I79f846c9d4777fc45abff220632a7afc80819d0a
Reviewed-on: https://chromium-review.googlesource.com/914129
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Danyao Wang <danyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536869}