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This CL adds a few tests to validate text extraction and accessibility
tree for the newly enhanced accessibility support for PDFs. The tests
added make use of an already available test file as well as add another
test file.

Bug: 981448
Change-Id: I821751efc9cf2c586c41ecdf29a071d95a52dd70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1817857
Commit-Queue: Mohit Bansal <mohitb@microsoft.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#702572}