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Blink team is planning to move DOM attributes into prototype chains.
DOM attributes will no longer be accessible as Object properties, e.g.
JSON.stringify will not see DOM attributes.  So WebRTC tests shouldn't
expect that they can stringify DOM attributes.

Fixes tests not to directly stringify DOM objects.

BUG=43394

Review URL: https://codereview.chromium.org/811653007

Cr-Commit-Position: refs/heads/master@{#311472}