Add a data source field for volumes.
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
Committed:
https://crrev.com/181fd73aa111fff257616435820a6392e309a5e5
Cr-Commit-Position: refs/heads/master@{#324429}