Change XRRigidTransform inverse method to attrib

Changed to match recent spec change. Given that the XRRigidTransform is
a static object once created having inverse as an attribute makes sense
because it's expected to return the same value every time it's called.

Bug: 948392
Change-Id: I667d694b58d8307316fe6a278672187c37df58af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1548615
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#647387}
13 files changed