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This was introduced in https://crrev.com/c/2142514 when the invalid
value being case to an EventDisposition enum was turned into a
base::Optional de-ref. This situation should never occur but the crashes
indicate it does.

This CL preverves old behavior to prevent the crash.

TBR=nzolghadr@chromium.org

Bug: 1069760
Change-Id: I412edf2bff202d68730a3374792f1dab16a91426
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145567
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758268}