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No change to logic, this just changes explicit calls to the
std::unique_ptr constructor to instead use make_unique() or implicit
conversion from nullptr. This is to conform with a chromium presubmit
check.

This makes MetricSample's constructor public for compatibility with
make_unique(), and documents the existing static methods are still the
preferred way to instantiate the class.

This also fixes some indentation problems found by 'git-cl format'.

Bug: 1010369
Test: git cl presubmit --upload
Change-Id: I734a48629c435a8c31fe15b8a8d3e44352dcae08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1834656
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702611}