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This CL introduced a new class extensions::UsbDeviceManager and merged
it with existing extensions::UsbEventRouter. This new class will proxy
all USB mojo requests in chrome.usb extension implementation.

Bug: 716628
Change-Id: Ie296e6682dedd938e59432dd3b35f2a92fc9a69a
Reviewed-on: https://chromium-review.googlesource.com/c/1424739
Commit-Queue: Donna Wu <donna.wu@intel.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626446}