Listen to UsbDeviceManager instead of UsbService in UsbChooserContext.

For UsbService::Observer, "OnDeviceRemovedCleanup" needs to be handled
after "OnDeviceRemoved". But for mojom interface UsbDeviceManager,
there is no way to provide such dependency among listeners.

As all users will be converted from UsbService to UsbDeviceManager,
to avoid the potential race condition, WebUsbServiceImpl is made
listen to UsbChooserContext in this CL.

Bug: 699790
Change-Id: I1e3b45196552e987b7422b127651e1fb01e1618b
Reviewed-on: https://chromium-review.googlesource.com/1210386
Commit-Queue: Donna Wu <donna.wu@intel.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590609}
11 files changed