Implement generalized file preparation utility function for apps.
The previous implementation worked only for Google Drive filesystem.
The new implementation in the current CL generally supports all
volumes accessible from Chrome OS file manager, in particular
including MTP devices and file systems provided by fileSystemProvider extensions.
BUG=367028
Review URL: https://codereview.chromium.org/
303133008
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274222 0039d316-1c4b-4281-b951-d872f2087c98