Move OnFileSystemMounted/BeingUnmounted to VolumeManager.
By this CL, Drive's mounting state is managed by VolumeManager. So, now
EventRouter needs to listen only VolumeManager to report "mounting" state
update.
Along with the change, the responsibility to create DriveIntegrationService is moved to VolumeManager from EventRouter.
BUG=279276
TEST=Ran unit_tests and tested manually.
Review URL: https://chromiumcodereview.appspot.com/
23477070
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@223029 0039d316-1c4b-4281-b951-d872f2087c98