Fix PermissionType enum in histograms.xml
Fix PermissionType enum in histograms.xml

388c056b78a6cf1f43520e6c44b7ffcc57e7de6c landed with an inconsistency.
Durable storage was 7 in C++'s PermissionType enum but 6 in the uma
label; MIDI snuck in as 6 while that patch was trying to land.

Review URL: https://codereview.chromium.org/1312463003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#345268}

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