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Mainly, this breaks out braille span classes and the EditableTextBase
class into their own files.  This removes lots of dependencies on code that
should only run in the content script in ChromeVox classic from the
ChromeVox next background script.

BUG=

Review URL: https://codereview.chromium.org/889593002

Cr-Commit-Position: refs/heads/master@{#313895}