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Per the WebXR Spec, the inputSources attribute is supposed to be a live
array whose input sources are updated in place.  Currently the code
creates a new array on every query.  This changes the internal hash map
of input sources to be an XRInputSourceArray object.

The XRInputSourceArray object is implemented with a backing HeapHashMap
so that the existing logic (i.e. looking up/setting/erasing by id) is
more performant than getting by the index.  This does have a side effect
of meaning that an input source's position in the inputSources array can
change, as it's ordering is based on the ordering of the HeapHashMap's
backing .Values store.

Bug: 966552
Change-Id: I21396ebbbbef6799a6bdd98791706c9159c9e061
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632872
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664463}