VR: Move UI-specific code into the UI for cleaner separation

This change makes the future introduction of a VR UI feature module
possible by cutting dependencies between VrShellGl and UI code.  This
includes creating a UI support shim to help avoid UI calls directly
to code that may not be exported to modules in the future.

BUG=

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:linux_vr;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ib2edbe31ca8c6fcbbfbd8facc9913d6068eda208
Reviewed-on: https://chromium-review.googlesource.com/1141934
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576187}
11 files changed