VR: Disable GL depth testing for browser and overlay

Depth testing is no longer needed, because the element tree defines draw
order. Furthermore, depth testing can negatively interfere with
rendering of planar elements.

BUG=

Change-Id: I0da0d3b30a8438c6a2ad0670c414ac41e4222f24
Reviewed-on: https://chromium-review.googlesource.com/762276
Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515827}
4 files changed