First set of WebUSB layout tests with Mojo service mocks.

This patch adds a minimal set of layout tests for the WebUSB API that
work by registering mock implementations of the Mojo services the Blink
implementation depends on. Since these mocks are implemented using the
Javascript Mojo bindings they can live right next to the tests that
depend on them.

BUG=492204

Review URL: https://codereview.chromium.org/1726943002

Cr-Commit-Position: refs/heads/master@{#377429}
6 files changed