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- $descendants for div's can be very expensive. For example, in Gmail, there is a focusable div that wraps almost the entire contents of the page. We end up generating Output for everything under the div and lag significantly when focus lands here.
Fix this by only taking the text content of divs as one large string.  $descendants had all output including rich formatting.
- live region output on alerts causes double output (and similar lag). For example, pressing '?' in Gmail triggers hundreds of runs inside of LiveRegion.
Fix this by ignoring live regions set on alert nodes.

TEST=navigate in Gmail.
BUG=672955
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2563173002
Cr-Commit-Position: refs/heads/master@{#437925}