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Previously watch ids were generated in the DeviceService. This CL
switches to do that in Blink side, more specifically NFCProxy is
responsible to generate/manage them, and still keeps their uniqueness
over a given Mojo connection of device.mojom.NFC interface.

This makes it easier for NFCProxy to track the set of active readers,
specifically the 2 problems below can get solved.

1.
  a. NFCReader#start() --> device.mojom.NFC.Watch(options), we'll get
     the watch id via the response callback.
  b. in DeviceService the NFCImpl Watch() generates WATCH_ID_XXX and
     passes it back via the WatchResponse callback.
  c. in DeviceService NFCImpl gets a matched NDEFMessage from a NFC tag
     and call device.mojom.NFCClient.OnWatch(WATCH_ID_XXX, ndef_message).
As NFC and NFCClient run on separated Mojo message pipes, the Mojo
messages sent by the above a and b do not have any ordering guarantee,
i.e. c may gets to NFCProxy earlier than b, then, because NFCProxy did
not get WATCH_ID_XXX yet, it can not find the corresponding reader
that should actually be the receiver of this NDEFMessage.

2.
  a. NFCReader#start() --> device.mojom.NFC.Watch(options)
  b. NFCReader#stop() --> can not call device.mojom.NFC.CancelWatch(id)
     because we did not get the watch id yet.
  c. the WATCH_ID_XXX for step a above comes back.
This just leaves the watch session of WATCH_ID_XXX long live in NFCImpl
without any chance to cancel it.

BUG=520391

Change-Id: I136ee6aab54eb464e5dd684ed34a2d738d2f5198
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730515
Commit-Queue: Leon Han <leon.han@intel.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683914}