PrivateAggregation: Optimize special case of `maxContributions`

This CL implements an optimization for per-context limits, which are
still gated by a feature and disabled by default.

We're making `ShouldSendReportDeterministically()` a little more clever,
with the goal of enabling Private Aggregation to send fewer null reports
without loss of privacy. Rather than requiring determinism whenever the
`std::optional<size_t> max_contributions` parameter has a value, we will
now additionally require that it would make the aggregatable report
stick out. This resembles the semantics of the `filtering_id_max_bytes`
parameter.

This CL should not contain any web-visible behavior changes, primarily
because none of the calling APIs have defined the `maxContributions`
field yet.

Bug: 376707230
Change-Id: Iee0d849170e728fd9cdc540daa83881ce96a21dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6183147
Commit-Queue: Dan McArdle <dmcardle@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1412280}