Enable WebUSB through the Experimental Framework.

Adds the APIExperimentEnabled annotation to all of WebUSB's IDL
interfaces. If the Experimental Framework is not enabled (or the
experiment key check fails) this API can still be enabled using the
--experimental-web-platform-features flag.

The USB interface is now always added to the local frame and WebUSB
notifications are enabled for all users (as the assumption is that only
sites with an EF key will actually be producing devices with the
necessary descriptors).

BUG=492204

Review URL: https://codereview.chromium.org/1671553004

Cr-Commit-Position: refs/heads/master@{#386201}
21 files changed