Enable WebUSB on Android.

This change finishes up the work done to remove #ifdefs disabling USB on
Android by making the WebUSB flag available on all platforms and
removing the #ifdef preventing the WebUSBClient from being provided to
the render frame on Android.

BUG=492204

Review URL: https://codereview.chromium.org/1493183002

Cr-Commit-Position: refs/heads/master@{#363826}
4 files changed