Web MIDI: Fix a broken UMA
Web MIDI: Fix a broken UMA

Size calculation was mistakenly wrong, and always 1 was reported.

BUG=578970

Review URL: https://codereview.chromium.org/1784643007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#380586}

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