[Memory Sheriff] Fix match --top-symbols
[Memory Sheriff] Fix match --top-symbols

Top symbols would count _total_ occurrences of any given symbol. What
usually matters is the number of suppressions a symbol occurs in, so
PrintTopSymbols now removes duplicate suppressions for a given symbol.

R=thestig@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/1694263005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#375766}

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