[vr] Stop hit testing in bindings.

With this change, we rely on hover states rather than hit testing to
trigger the visibility of the content frame.

Bug: None
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: I3b800073ea6233cba479ffb6fbd3ea7e1f37ffa4
Reviewed-on: https://chromium-review.googlesource.com/1006224
Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549732}
3 files changed