Pepper: FileIO holds FileSystem ref with scoped_refptr<Resource>.

1) Use a scoped_refptr to keep the FileSystemResource alive.
ScopedPPResource isn't safe for this purpose.
2) Make sure that the FileIOResource always sends a close message, so
the FileIO host can unhook from the FileSystemHost before the latter is
destroyed. Due to refcounting, the destruction order is in some cases not
what we want.

R=teravest@chromium.org
BUG=330809,330859

Review URL: https://codereview.chromium.org/121813002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242646 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242646