Add Typemapping for WebXR mojom types to Blink

This is the first step to convert the types in the vr_service.mojom to
common gfx.mojom geometry types.  By adding typemaps from the gfx.mojom
geometry types on the blink side, we can have cleaner and consistent
code across both sides of the mojom pipe.

Bug: 977008
Change-Id: I1eafdc938be9d5dc7e67ecd5c964d07a90bc6d29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1668569
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672665}

• third_party/blink/renderer/modules/xr/xr_bounded_reference_space.cc[diff]
• third_party/blink/renderer/modules/xr/xr_input_source.cc[diff]
• third_party/blink/renderer/modules/xr/xr_session.cc[diff]
• third_party/blink/renderer/platform/mojo/blink_typemaps.gni[diff]
• third_party/blink/renderer/platform/mojo/geometry.typemap[diff]
• third_party/blink/renderer/platform/mojo/geometry_struct_traits.cc[diff]
• third_party/blink/renderer/platform/mojo/geometry_struct_traits.h[diff]
• third_party/blink/renderer/platform/mojo/geometry_struct_traits_test.cc[diff]
• third_party/blink/tools/blinkpy/presubmit/audit_non_blink_usage.py[diff]