Make XRPlane return XRSpaces instead of handing out XRPoses

Currently, the pattern to obtain XRPoses in WebXR is to invoke
`XRFrame.getPose(space_to, space_from)` method - XRPlanes were
not following the pattern and returning XRPoses directly from
`XRPlane.getPose(space)`. This CL fixes that inconsistency -
planes will now return instances of XRSpace that can be used
to obtain the poses from a frame.

Change-Id: Ie656bcca50fe7118d5e2ffeb94e86e101b6e7f82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678440
Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672724}
8 files changed