This change adds a WorkerNavigatorUSB supplement for WorkerNavigator.
The WorkerNavigatorUSB interface contains a usb attribute that returns
a USB object. The constructor for this USB object was also changed to
use an ExecutionContext* parameter to allow a worker to properly
initialize the object.

This change also updates the global interface listing tests to reflect
the new attribute that WorkerNavigator contains and adds web platform
tests to check that the WorkerNavigator does indeed have the usb
attribute defined in the supported contexts.

Due to a bug (https://crbug.com/839117) with the WebIDL bindings
generator, the global-interface-listing-service-worker.html test
exposes the usb attribute of WorkerNavigator. Therefore, a check is
performed in WorkerNavigatorUSB to return a null pointer if the
execution context is a ServiceWorkerGlobalScope. Since the intended
behavior for navigator.usb in a service worker is to be undefined, the
web platform test for this checks that condition instead.

Bug: 837414
Change-Id: I3ca63c9ae0b85e5425134024d2fdd9b29ef7d8c3
Reviewed-on: https://chromium-review.googlesource.com/1024724
Commit-Queue: Ovidio Henriquez <odejesush@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Chong Zhang <chongz@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557272}