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This is a reland of a4418459. The
original CL was causing crashes due to UaF on the Scorer object.
This CL fixes that issue by performing the thread hop within Scorer.
Since the returning callback on the PhishingClassifier is guarded by
a WeakPtr, this should be more safe.

Original change's description:
> Move visual feature scoring to helper thread
>
> This CL calls the GetMatchingVisualTargets in a worker thread, since
> the process of scoring visual features takes longer than expected
> and was blocking interaction.
>
> Fixed: 1121375
> Change-Id: I8f3e33d20f812645a72adfc16fe33241b49a6ff4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2372869
> Commit-Queue: Daniel Rubery <drubery@chromium.org>
> Reviewed-by: Bettina Dea <bdea@chromium.org>
> Reviewed-by: Varun Khaneja <vakh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#801907}

BUG=1121375,1122534

Change-Id: Ifedb94ad8b613a2ba7377bb5bc7795e657a94512
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386380Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Bettina Dea <bdea@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805408}