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- when firing an active descendant change event on a content editable, we were
  serializing line breaks which took approximately ~600MS. The line break
  calculation was not only slow, but wrong (got stuck and put repeated new lines
  at the same offset). Unfortunately, <textarea> still needs line break
  information, so keeping for the !richlyEditable but editable case.

- Sheets sends empty strings through the ChromeVox Classic speak api. This
  should never be allowed. It causes ChromeVox output to be flushed and stopped.

- value changes get fired on root web area. Root web areas usually have '' as
  their values. This also caused output to be prematurely cut off.

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ide73784a23db7bc92aef51de917801dab439dbba
Reviewed-on: https://chromium-review.googlesource.com/964781Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543589}