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This is a reland of 653f075d
(After deflaking the test)

Original change's description:
> Clear the subresource redirect hints when navigation is started
>
> RenderFrames could be reused for same-origin navigations. This causes
> the hints from previous navigations to be used in next navigation
> until the new hints fetch finishes and updates the hints. This CL clears
> the hints.
>
> Unfortunately this causes race conditions between the image fetch and
> the hints update, which causes images to not redirected to compressed
> versions. So the image fetches are delayed a bit to circumvent this.
>
> Bug: 1051283
> Change-Id: Ie9e5351e68c0081ab9dbfe23faed2c3c5e8e4d42
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2056207
> Commit-Queue: rajendrant <rajendrant@chromium.org>
> Reviewed-by: Michael Crouse <mcrouse@chromium.org>
> Reviewed-by: Tarun Bansal <tbansal@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#742326}

TBR=mcrouse@chromium.org

Bug: 1051283
Change-Id: I0336295bc6120c16e02dd28f787f2edcfe37e9fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2065984Reviewed-by: rajendrant <rajendrant@chromium.org>
Commit-Queue: rajendrant <rajendrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743275}