Plumb texture target for WebXr local textures

Right now target is always hardcoded to be TEXTURE_EXTERNAL, because
SurfaceTexture case requires it. For SharedBuffers case it's preferred
to use TEXTURE_2D, so we need some plubming.

This CL just adds plubming and doesn't have any behavior change.

Bug: 360147123
Change-Id: I1af3c3b16828f4c797141c0ebc7eecbad935c0e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5786749
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1345513}