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A few APIs validate arguments against a signature specified in an event
call, rather than as an API method. Allow this validation to happen
natively when native bindings are enabled by adding a JS utility to add
a custom signature and validate against it.

Use this for certificateProvider, printerProvider, and webRequest custom
bindings instead of the JS validation.

Add unittests for the new JS utility method.

Bug: 810487

Change-Id: Ifbc8480ae15acbdaa9ecdffcfb7bfe159e201510
Reviewed-on: https://chromium-review.googlesource.com/914808Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545806}