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The TestPrintBackend class has been a minimal implementation of a
PrintBackend that could be used in a limited fashion for focused unit
tests.  It did not behave in a consistent fashion between the various
interfaces; for example, setting a new default printer would not be
reflected when getting PrinterBasicInfo.

The out-of-process print driver feature is intending to use this class
in a more general manner, for use of fake printer drivers in browser
tests.  This makes it desirable to make this class be better behaved.

Introduce unit tests specifically for TestPrintBackend, to provide
confidence for use by larger browser tests.

Ensure that the same mapping of available printers is used across
the PrintBackend APIs, so that calls to get printer info or
capabilities would be guaranteed available for any printer identified
by EnumeratePrinters().  Also ensure that the default printer is kept
consistent when the environment is generated or later modified.

Updated PrintBackend::PrinterBasicInfo class to have a
fully-parameterized constructor and equality operator to assist with
unit tests here as well as in the forthcoming tests for later OOPPD
changes.

Bug: 809738
Change-Id: Ia35e0c99bdb0eba8faa48dfc955d73a816e7d84d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463559
Commit-Queue: Alan Screen <awscreen@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827566}