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Especially for FSP, we'd like to know type of a volume, as the way they are
going to be displayed in Files app depends on it. Eg. SOURCE_FILE volumes will
be auto opened once they are mounted. SOURCE_NETWORK file systems will have
a button for configuration.

TEST=unit_tests, browser_tests
BUG=474146

Review URL: https://codereview.chromium.org/1055183003

Cr-Commit-Position: refs/heads/master@{#324429}