MediaRecorder: content browsertests fix a few lint warnings in mediarecorder_test.html
MediaRecorder: content browsertests fix a few lint warnings in mediarecorder_test.html

This CL fixes a few lint warnings scattered around
mediarecorder_test.html. No new functionality added.

BUG=575323
Test=On linux host and trybots.

Review URL: https://codereview.chromium.org/1565343002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368643}

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