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XRSession has a collection of V8 callbacks that it will call for
animation frame requests.

These callbacks were detected as unreachable from V8's perspective,
even though XRSession was alive from blink's perspective.

The fix is to make XRSession an ActiveScriptWrappable, so it is a
root object from V8's garbage collection while there are active
callbacks.

In discussion about this bug, it was pointed out that a couple places
we keep objects on the stack might also cause V8's garbage collector
to think wrappers are unreachable, so I now keep those objects alive
with explcit traceable references.

BUG=870403

Cq-Include-Trybots: luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I54cfda8597439c0a988acd918a1fcc6904cca340
Reviewed-on: https://chromium-review.googlesource.com/1166187
Commit-Queue: Bill Orr <billorr@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581515}