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Previously NDEFReader#scan() returns void and an NDEFErrorEvent will be
dispatched in case that the scan operation cannot be started
successfully.

This is not a well accepted pattern, and also to align with
NDEFWriter#push(), this CL makes NDEFReader#scan() return a Promise
instead.

Note that now NDEFErrorEvent is only used to notify Mojo disconnection,
a follow-up CL will remove it completely by using ErrorEvent instead.

The spec change:
https://github.com/w3c/web-nfc/pull/398
https://github.com/w3c/web-nfc/pull/432

BUG=520391

Change-Id: I1477258ab70f7e40da31ea8795d63125b6a13af0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899586
Commit-Queue: Leon Han <leon.han@intel.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716904}