To be clear -- in UiScene, we filter out our elements based on IsVisible which does not account for local opacity.

In addition to this, we should not render any GradientQuad that has transparent colors.  This will eliminate rendering for button backgrounds (etc) when they aren't hovered.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b2a2ee995b073be1fa5c647953e3cdcb25de4bb8

commit b2a2ee995b073be1fa5c647953e3cdcb25de4bb8
Author: Christopher Grant <cjgrant@chromium.org>
Date: Thu Apr 12 13:17:58 2018

VR: Do not render transparent rectangles

Rects with a local opacity of 0 should not render.  In addition, any
quad with transparent color should also not render.

This change eliminates the overhead of drawing the content reposition
frame if it's not visible.

BUG= 831625 

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I3245005ab448a6958bc786603568431850cb1bdb
Reviewed-on: https://chromium-review.googlesource.com/1007494
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550168}
[modify] https://crrev.com/b2a2ee995b073be1fa5c647953e3cdcb25de4bb8/chrome/browser/vr/elements/rect.cc
[modify] https://crrev.com/b2a2ee995b073be1fa5c647953e3cdcb25de4bb8/chrome/browser/vr/renderers/gradient_quad_renderer.cc

Turns out this yielded a significant performance increase.  The reasons aren't well-understood, but, bonus!