Web MIDI: remove old Windows backend
The new backend supporting dynamic instantiation on Windows was rolled
out to the stable channel via a field trial flag.
This new backend solves some long-living problems, e.g., grabbing
devices forever, battery concerns, and stability.
Since the feature was enabled by default, and have worked well,
I want to remove the old backend now.
Another change that renames new backend will follow.
BUG=672793
Review-Url: https://codereview.chromium.org/2841493003
Cr-Commit-Position: refs/heads/master@{#466904}
5 files changed
