Hide video tags and canvases for WebRTC multi-renderer test case.

Hide video tags and canvases for WebRTC multi-renderer test case.

This particular test case created new video tags and canvasses
dynamically, which rendered video on the page. This, unfortunately,
is too expensive for some slow bots, so we need to hide them to
prevent them from rendering.

This is why the default video tags and canvasses in the test page are
hidden. We can still extract pixel values from them even if they're not
rendering at the top level, so the test still works.

TBR=kbr@chromium.org
BUG=523152,523152
CC=perkj@chromium.org,mcasas@chromium.org

Review URL: https://codereview.chromium.org/1302293003

Cr-Commit-Position: refs/heads/master@{#345327}