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This is a reland of a3dd29b2.

Original change's description:
> Bounce Blob URL revoking through the UI thread to give navigation a chance.
>
> Navigation IPCs are handled on the UI thread so if a blob URL is revoked
> immediately after navigating the revocation will almost always happen before
> the navigation code has had a chance to post a task back to the IO thread to
> actually perform the request. With this change revocation is hopefully
> delayed just long enough to make navigation work again.
>
> Bug: 807639
> Change-Id: I2afde86dd203c61cbb03cc02ac62ec21c7e7d704
> Reviewed-on: https://chromium-review.googlesource.com/899666
> Reviewed-by: Jochen Eisinger <jochen@chromium.org>
> Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#534119}

TBR=mek@chromium.org,jochen@chromium.org

Bug: 807639,808759
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I5033422d57ac7672e761045f7871c6aed29368b9
Reviewed-on: https://chromium-review.googlesource.com/900191
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534257}