Reland of Refactor WebUSB LayoutTests to separate out the Mojo service mocks (patchset #1 id:1 of https://codereview.chromium.org/2791573002/ )

Reason for revert:
Fixed the leak.

Original issue's description:
> Revert of Refactor WebUSB LayoutTests to separate out the Mojo service mocks (patchset #1 id:1 of https://codereview.chromium.org/2775323003/ )
>
> Reason for revert:
> Breaks leak bot:
>
> https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Linux%20Trusty%20Leak/builds/3557
>
> * usb/test-polyfil.html
> * usb/usb-connection-event.html
> * usb/usb.html
> * usb/usbDevice-iframe.html
> * usb/usbDevice.html
>
> seem to be leaking :(
>
> Original issue's description:
> > 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}
> > Committed: https://chromium.googlesource.com/chromium/src/+/e1edfbbb4b3a192e210c031628e4f12aa93057cd
>
> TBR=yzshen@chromium.org,reillyg@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=705734
>
> Review-Url: https://codereview.chromium.org/2791573002
> Cr-Commit-Position: refs/heads/master@{#461045}
> Committed: https://chromium.googlesource.com/chromium/src/+/245d44d8347cb000d4d78b46b9f120e3fc9f10d3

TBR=yzshen@chromium.org,ortuno@chromium.org
BUG=705734

Review-Url: https://codereview.chromium.org/2788463005
Cr-Commit-Position: refs/heads/master@{#462203}
9 files changed