fix(dialog): fix focus trap sometimes not working

[ErinCoughlan]

@jelbourn @KarenParker @gmoothart @DerekLouie Please review.

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[ErinCoughlan]

I signed it! as part of the Google Corporation.

[googlebot]

CLAs look good, thanks!

[jelbourn]

LGTM

Can you squash your commits with git rebase -i and update the commit name to match our pattern? It would be something like
`fix(dialog): fix focus trap sometimes not working"

[ErinCoughlan]

Fixed commit message and squashed the commits.