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Files app fetches information whether a volume is read only or R/W from
file_manager::VolumeManager, which used to always have the isReadOnly option
set to true for any provided file system.

However, since R/W operations are now under development, we should be able to
mark a volume as R/W.

In order to achieve that, a "writable" option has been added to the File System
Provider API mount() call. The argument is by default false, so file systems
are by default read only.

TEST=unit_test, browser_tests: *FileSystemProvider*Writable*
BUG=391362

Review URL: https://codereview.chromium.org/372163006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282890 0039d316-1c4b-4281-b951-d872f2087c98