Use /var/run/crouton to mount chroots.

Avoids the need to --make-unbindable, prevents polluting the file system, and
simplifies logic.
Properly unmount old chroots that are still mounted.

Avoids issues with upgrading.
Double-unmount for old-style chroots.
Properly handle unmounted chroots.
Use return instead of continue.
Enforce prefix existence
Remove redundant set +e
Unmount chroots before unmounting the prefix.
Fix using addtrap in tests.
Add /media tests.
Fix random background test failure.
Fix path in enter-chroot, unmount-chroot, and installer messaging.
Fix chroot setup script validity test, and delete the script on failure.
Add cros parameter to croutoncycle.
Make sure exit part is run in croutonxinitrc-wrapper, even if we press Ctrl-C.
Chromium extension.
WebSocket server for communication for Chromium extension.
Scripts for communication with Chromium extension (clipboard synchronization and URL handling).
Integrate extension into crouton's framework.
Wait for child so that we do not leave sha1sum and base64 zombies.