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See bug for details. This patch makes it so that we fire
the events  IA2_EVENT_TEXT_INSERTED and IA2_EVENT_TEXT_REMOVED
when the children referenced by the embedded object characters
changed, not just when the text changed.

This patch also avoids incorrectly firing these same two
events on a newly-created node.

BUG=425861

Review URL: https://codereview.chromium.org/859133003

Cr-Commit-Position: refs/heads/master@{#314280}