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Reland #2 "Add field trial testing config for BoostThreadPriorityOnLibraryLoadingAndBackgroundMode."

This is a reland of 9d6a01ba

//base now supports ::GetThreadPriority() returning 3, so this
shouldn't cause problems on bots. https://crrev.com/712564

Original change's description:
> Add field trial testing config for BoostThreadPriorityOnLibraryLoadingAndBackgroundMode.
>
> This experiment enables:
> - Use THREAD_MODE_BACKGROUND_BEGIN/END to set the priority of
>   background threads. This affects memory and IO priority in addition
>   to CPU priority.
> - Increase thread priority to "normal" in scopes where background
>   threads load modules.
>
> The expectation is that this will improve responsiveness of Chrome
> (input delay, first contentful paint), without regressing jank.
>
> We are adding the testing config in preparation for enabling this
> via Finch in M79.
>
> Bug: 973868
> Change-Id: I467cae12503e3ad724b665f3aebdaeb94e1325c9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1835023
> Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
> Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
> Auto-Submit: François Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#705960}

TBR=rkaplow@chromium.org

Bug: 973868
Change-Id: I26127cc8e545a1f187a806ed5247c6b91f2f4db0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899790Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712713}