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Providing extensions will very often mount provided file systems on startup.
When an extension is installed, the background is executed, and hence the file
system registered.

Since providing extensions should be event pages, they will also register
all of the request events, so the extension is woken up once there is a request
sent to it.

However, the background page is not run after a reboot. All of the registered
events are remembered in preferences, but mounted file systems not. As a result
after a reboot, the file systems are lost.

To overcome this issue, this CL introduces storing mounted file systems to
preferences, so they are remounted automatically after a reboot, once the
extensions are loaded. This is consistent with remembering registered
event handlers in preferences.

Note, that if the extension is gone after a reboot, then remounting will not
be performed, since it is done after the extension is loaded.

All of the mounted file systems are written to preferences during shutdown.

TEST=TBD
BUG=248427

Review URL: https://codereview.chromium.org/295413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274214 0039d316-1c4b-4281-b951-d872f2087c98