Adds VolumeManagerWrapper.

VolumeManagerWrapper is a thin wrapper for VolumeManager, which handles drive
enable/disable state on UI side (hidding Drive Volume if it shouldn't be shown), and initialization state.

BUG=268817
TEST=Ran browser_tests --gtest_filter="*FileSystemExtensionApiTest*:*FileManagerBrowserTest*:*FileBrowserPrivateApiTest*" and tested manually.
R=mtomasz@chromium.org

Review URL: https://codereview.chromium.org/23439013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225421 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225421