Refactor WebUSB LayoutTests to separate out the Mojo service mocks

In preparation for being able to upstream the existing WebUSB tests to
the Web Platform Tests repository this patch splits out the Mojo
service mocks into webusb-test.js. This file is a polyfil for a
fictional "WebUSB Test API" that could in theory be implemented by any
browser (an idea borrowed from a proposed plan for Web Bluetooth).

BUG=705734

Review-Url: https://codereview.chromium.org/2775323003
Cr-Commit-Position: refs/heads/master@{#460978}
9 files changed