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- ! allowed by a tts property will annotate the output with that property
- $descendants(args...) where args... is a comma delimited list of roles, will compute output for the node's descendants that are in the set of roles.
- apply these two new functions to compute utterances for alerts.

TEST=navigate to a page with an alert like
html5demos.com/geo.
BUG=382650

Review URL: https://codereview.chromium.org/816093002

Cr-Commit-Position: refs/heads/master@{#314688}