This reverts commit 74fd352e51322e823070088c7d5c72e911966580.

Reason for revert: Moving behind experimental feature flag until web compat can be assessed.

Original change's description:
> Cache Storage: Check for duplicate entries in Cache.addAll().
>
> This implements step 4.2.3 of the BatchCacheOperations algorithm:
>
> https://w3c.github.io/ServiceWorker/#batch-cache-operations-algorithm
>
> Bug: 720919
> Change-Id: I679f786441b813ed816a183522021c417f4ab7b8
> Reviewed-on: https://chromium-review.googlesource.com/1162362
> Commit-Queue: Ben Kelly <wanderview@chromium.org>
> Reviewed-by: Joshua Bell <jsbell@chromium.org>
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Reviewed-by: Victor Costan <pwnall@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#582738}

TBR=jsbell@chromium.org,kinuko@chromium.org,pwnall@chromium.org,wanderview@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 720919
Change-Id: I08736e4ec638a65c48f329966f378698a6d9b045
Reviewed-on: https://chromium-review.googlesource.com/1178502
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583849}