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The XRSystem is currently provided its document's ukm::SourceId in its
constructor, which is then stored in a member variable. Instead, we can
simply get it from the document/execution context as needed.

Follows from comments in crrev.com/c/2267639.

Bug: 1100349
Change-Id: Ie89c7fa55ee27aa07cc799666c83443122d721fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2451384Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814353}