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This change updates the WebUSB WebIDL files to match the recently merged
spec pull request #142 by hiding interfaces which aren't useful in
insecure contexts.

Change-Id: I98625b2634e452c775601525300672d9440ddbcc
Reviewed-on: https://chromium-review.googlesource.com/1137435
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576316}