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This required to create a custom callback class as CallbackPromiseAdapter
does not work well with dictionaries.

This is also updating the media-capabilities.idl file in WPT, based on:
https://raw.githubusercontent.com/tidoust/reffy-reports/master/whatwg/idl/media-capabilities.idl

Bug: 907704
Change-Id: I13bb3c43a7461398f7866f033527bd3ce1bed140
Reviewed-on: https://chromium-review.googlesource.com/c/1347001
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610655}