Add Transient FileSystem which goes away after its snapsot file is gone

Add Transient FileSystem which goes away after its snapsot file is gone

- Adding a new filesystem type, kFileSystemTypeForTransientFile,
  which is supposed to only contain a single temporary file
  and must be go away after the last reference of the snapshot file
  for the temporary file is dropped.

I'm planning to use this filesystem for creating Blobs for
manual conflict handling API.

BUG=177159
TEST=TransientFileTest.TransientFile

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=196670

[kinuko, 2013-04-25 06:29:27]

Can you take a look?

[tzik, 2013-04-25 08:06:40]

lgtm

[kinuko, 2013-04-26 08:11:03]

Committed patchset #1 manually as r196670.