Add chrome.fileSystem.GetVolumeList().

This CL adds a method to list available volumes. It's available only to kiosk
apps running in kiosk session and to some whitelited component apps/extensions.

[email protected]  # chrome_extensions.js reviewed at cr/90486340.
TEST=browser_tests: *KioskTest*GetVolumeList*,
     *FileSystemApiTestForRequestFileSystem*GetVolumeList*
BUG=440674

Review URL: https://codereview.chromium.org/1029803004

Cr-Commit-Position: refs/heads/master@{#324040}
19 files changed