Revert "Enable MidiManagerWinrt by default on Windows 10"

Make MidiManagerWinrt back behind a Finch flag disabled by default.

Reverting CL https://codereview.chromium.org/2392263002/ while preserving
strings and histograms for the "disabled" flag. Revert this CL to enable it by
default again.

BUG=645403,659458

Review-Url: https://codereview.chromium.org/2450033003
Cr-Commit-Position: refs/heads/master@{#428608}
5 files changed