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Phase 1 of ChromeVox custom annotations project.

This change adds a new file to the ChromeVox codebase called
node_identifier.js. This file contains objects and methods used to
uniquely identify AutomationNodes. Several basic tests have also been
implemented in node_identifier_test.js.

This file is being added to support ChromeVox custom annotations.
For more details on that project, please see the following design

document: 
https: //docs.google.com/document/d/1K6Sg0wLOjUdz7ycJqxPnFWNvie7yaJ3enlss10cXKqM/edit
Change-Id: Id5c60ec9257bef0738366d045100915e26d19e6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992594
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734949}