[Protected Audiences] Rework seller signals cross-origin delay logic.

Previously, the SellerWorklet called a Pause() method on its
TrustedSignalsManager until it had verified the seller would let it
send requests cross-origin, at which point it resumed them.

This CL removes TrustedSignalsManager::Pause(), and instead has the
SellerWorklet delay sending requests to the TrustedSignalsManager until
it has validated the seller is ok with sending the information
cross-origin. This puts fewer requirements on the TrustedSignalsManager
(Like the expectation it's only used by a single SellerWorklet).

When the SellerWorklet learns it can send requests cross-origin, it
also now immediately flushes all pending seller signals requests,
rather than adhering to the send request delay or relying on tracking
the most recent SendPendingSignalsRequests() call.  While we could
respect those, it doesn't seem worth the effort.

Bug: None
Change-Id: Iafdfe9fc05d2c501a573aba32a222ef4781195e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5522414
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: mmenke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1298786}