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This removes the confusing code in content/renderer/accessibility that
serializes updates from Blink's accessibility tree, and replaces it with
calls to the new cleaner and fully-unit-tested AXTreeSerializer class.

BUG=316726

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=261099

Review URL: https://codereview.chromium.org/125763003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@261408 0039d316-1c4b-4281-b951-d872f2087c98