Make VR Device Eye Params optional

It does not make sense to have eye params on magic window only devices,
so this makes them optional in VRDisplayInfo.

Bug: 695937
Change-Id: Ib96f1c1a351af87739b4b419bba561d94ce96857
Reviewed-on: https://chromium-review.googlesource.com/807725
Commit-Queue: Anna Offenwanger <offenwanger@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522682}
8 files changed