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This is a reland of 44fcad8b

It was speculatively reverted for a build failure which turned out
to be something else.

Original change's description:
> Fix Shared Buffer 64 bit truncations
>
> Segments are always 4k and indexed by a WTF::Vector
>
> BUG=879657
>
> Change-Id: Ibe922cc6736fed0ba04e99aee0f04eb470f7fecf
> Reviewed-on: https://chromium-review.googlesource.com/c/1355884
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#612659}

TBR=haraken@chromium.org

Bug: 879657
Change-Id: I60299fc856a6dc987c8f4460b2bf3a339bcb5837
Reviewed-on: https://chromium-review.googlesource.com/c/1357302Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612727}