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In this CL:
1) Add the HidManager and HidConnection interfaces in hid.mojom, and
   implements those mojo interfaces in //device/hid.

2) Convert the clients in //extensions to use hid mojo interfaces.

3) Change the type of HidUsageAndPage::usage_page to uint16_t.

4) Rewrite the hid_apitest base on the new added mojo interfaces.

TODO:
1) Move client library files into device/hid/public/cpp.
2) Host HidService by DeviceService instead of DeviceClient.
3) Mojofy //device/u2f Or just move it into DeviceService.

BUG=728223

Change-Id: I1de1b5211f7ef37f54d4bc18c7995a3b47cc7da2
Reviewed-on: https://chromium-review.googlesource.com/648949Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Ke He <ke.he@intel.com>
Cr-Commit-Position: refs/heads/master@{#502109}