metrics: Reserve instead of resize when reporting leaks
metrics: Reserve instead of resize when reporting leaks

The reports are added to the vector of reports using push_back(). The vector
should initially be empty, but given enough capacity. It should not be resized.

BUG=chromium:382705
TEST=none

Review URL: https://codereview.chromium.org/1823313002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#382765}

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