Add SchedulePollEvents and PollEvents

Since WebVR spec already has VRDisplayEvent and definess several types of
events. Devices needs to be queried to get status and generate events.

These two methods enable the mechanism to query vr device. VRDeviceManager queries devices by
invoke PollEvents and vr device will implement PollEvent to do this.

BUG=389343
R=bajones@chromium.org

Review-Url: https://codereview.chromium.org/2266793002
Cr-Commit-Position: refs/heads/master@{#414970}
4 files changed