Capture from DOM elements: WPT idl tests

I just found the power-of-idlharness-tests; this CL
uses them for mediacapture-fromelement (now that WebKit
has landed capture from canvas, it's even more widespread).

(Inspired by https://crrev.com/2752383002/ WebUSB idlharness tests)

BUG=702862

Review-Url: https://codereview.chromium.org/2755293002
Cr-Commit-Position: refs/heads/master@{#458172}
2 files changed