Modify XRSession so that session geometry is only sent when it changes

Making it so that instead of receiving geometry every frame, ar_core
only gets geometry (frame size and rotation) when it changes. This saves
some processing every frame, and allows the GetFrameData interface to be
made uniform for AR and VR.

Bug: 856259,846521
Change-Id: I5828f57e1292f932c68148ac1e6b898f966b8d00
Reviewed-on: https://chromium-review.googlesource.com/1115776
Commit-Queue: Anna Offenwanger <offenwanger@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574611}
12 files changed