PA: anticipatory processes for InRendererAuctionProcessManager

Implement MaybeStartAnticipatoryProcess method for
InRendererAuctionProcessManager. This method will allow starting
processes at the beginning of the auction, in anticipation of needing
them later. Only processes that are required due to site
isolation will be started. This was previously implemented for
DedicatedAuctionProcessManager here:
https://chromium-review.googlesource.com/c/chromium/src/+/5899853

To make testing easier, move some of the functions useful for
testing anticipatory processes to a new base class,
AuctionProcessManagerTestBase.

Bug: 365528726
Change-Id: I02c9a42c0d7b4919def768ba5d0134945ece3438
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5933167
Commit-Queue: Abigail Katcoff <abigailkatcoff@chromium.org>
Reviewed-by: mmenke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1369657}