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This implements the ProvideIndependentMetrics API within the structured
metrics provider, ready to upload independent metrics. We can't actually
enable this yet, because of issues described in the TODO.

Unit tests to come once a related CL is also landed.

Bug: 1148168
Change-Id: I4fb739c7cbb2ecea54da82fe8382140cdf562946
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2543882Reviewed-by: Rachel Wong <wrong@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828589}