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This change reduces string copies in base::HistogramTester by changing
its APIs to take base::StringPieces instead of const std::string&. The
majority of callsites pass string literals to the HistogramTester APIs,
resulting in a deep copy with the old implementation.

Furthermore, this change updates the underlying std::maps to use the
heterogeneous std::less<> comparator, allowing the direct comparison of
base::StringPiece with std::string.

Bug: 384011
Change-Id: I6a8dd3e76280162b6046eac63a3d1ac84ed2ee6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1887612Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710454}