Fix raw_ref to Traceable type in XRHandIterationSource

raw_ref<T> where T is a traceable type is unsafe for the GC and should
be avoided. This CL replaces the raw_ref with a safe reference.

Bug: 331100928
Change-Id: I3f8a89d5deb1409eea598871f16f763f8c6d753a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5393061


Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1278745}