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The identifiability metrics reporting APIs should be able to consume
digestible parameters directly without passing it through a non-typesafe
conversion. In addition, the generated "digests" aren't really digests
in the sense that the output depends quite directly on partitioning of
the input, something that shouldn't affect digests.

In order to account for these, this CL introduces a concept of a
IdentifiabilityToken which can automatically consume supported data
types. It is a 'sample encoding' rather than a digest and cannot be used
as an input for the purpose of constructing stateful digests, because by
design it is not a digest.

From the perspective of a call site, these changes mean that the
invocation for recording a metric will go from this:

     IdentifiabilityMetricBuilder(
         base::UkmSourceId::FromInt64(document->UkmSourceID()))
       .Set(IdentifiableSurface::FromTypeAndInput(
                IdentifiableSurface::Type::kWebFeature,
                static_cast<uint64_t>(feature)),
            IdentifiabilityDigestHelper(value))
       .Record(document->UkmRecorder());

To:

     IdentifiabilityMetricBuilder(document->UkmSourceID())
       .SetWebFeature(feature, value)
       .Record(document->UkmRecorder());

Bug: 973801
Change-Id: I326bc5a4ead5efbb3a04bf8314692a151fa082f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207291
Commit-Queue: Asanka Herath <asanka@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Caleb Raitto <caraitto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775313}