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Previously we had an assumption that the event source from Android is
serialized in event node data. But it can be incorrect in some rare
cases.
This change prevents Chrome crash on such a case.

AX-Relnotes: n/a.
Bug: b:159861631
Test: AXTreeSourceArcTest
Change-Id: I22308ba29e23ff1de9271661443ceb7f67e79275
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2265895
Commit-Queue: Hiroki Sato <hirokisato@chromium.org>
Reviewed-by: Sara Kato <sarakato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782784}