VolumeMountWatcherWin: Reorder some operations

StorageMonitor is marked initialized before it has processed all the attach events for the initially available devices.

BUG=

Review URL: https://codereview.chromium.org/25261003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227297 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227297