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Use the PRETTY_NAME variable from /etc/os-release (or
/usr/lib/os-release) to get the "pretty operating system name in a format
suitable for presentation to the user" as the standard says.

This could allow us to drop a requirement on lsb_release binary on
Linux. The os-release is presented on most new distribution like Fedora,
RHEL 7, CentOS 7, Ubuntu 12.04+, Debian Wheezy+.

More information could be found on
https://www.freedesktop.org/software/systemd/man/os-release.html.

Bug: 420439
Change-Id: If4fc3cd3d22507e47b32d6184ac6ef4ae4b4ff3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1156504
Commit-Queue: Tomáš Popela <tomas.popela@gmail.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Tomáš Popela <tomas.popela@gmail.com>
Cr-Commit-Position: refs/heads/master@{#695280}