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}