Add a $joinedDescendants output specifier.

In some live regions, text gets separated into chunks but should rather be concatinated.

The makes the speech natural and not pause after each chunk.

TEST=chromevox_tests Output*.Join*

Review URL: https://codereview.chromium.org/1801363003

Cr-Commit-Position: refs/heads/master@{#382149}
4 files changed