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- Add the floor, ceiling and grid quads.
- Add the content backplane to avoid reticle jumping around the edges.
- Configure the scene reticle distance limit based on content.

BUG=715573
TEST=Verified background on regular page; no background in WebVR.

Review-Url: https://codereview.chromium.org/2844313002
Cr-Commit-Position: refs/heads/master@{#467814}