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Add browser tests to make sure PdfNupConverterClient works as expected:
- 2-up conversion for a 3 page document generates 2 page output.
- 4-up conversion for a 3 page document generates 1 page output.
- Garbage data input results in a conversion error.

Bug: 1004529
Change-Id: Ie31397dc0b9eb9d098763de3395f99bf69846889
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808581
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697409}