VR: Make VrWebContentsObserver platform independent

Bug: 891495
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: I6b4aa77dc0c81b79afe681c31ed2b4a06f19dc37
Reviewed-on: https://chromium-review.googlesource.com/c/1257816
Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Commit-Queue: Aldo Culquicondor <acondor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596201}
6 files changed