Implement generalized file preparation utility function for apps.

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
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=274222

[kinaba, 2014-06-02 07:25:14]

PTAL.

Note that the implemented behavior (if exists=>ok, else =>create) is
slightly mismatching with the function name because it doesn't check
whether or not the file is "writable".

This, however, is intentional and the name of the function is going
to change in https://codereview.chromium.org/306073004/
(See also the comments in the attached document why the change is intended.
In short, we are phasing out the explicit "writable" checks and
making it an error at the point where the actual write operation is issued.)

[mtomasz, 2014-06-02 07:43:23]

lgtm

[kinaba, 2014-06-02 08:15:33]

The CQ bit was checked by kinaba@chromium.org

[commit-bot: I haz the power, 2014-06-02 08:16:04]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/kinaba@chromium.org/303133008/50001

[commit-bot: I haz the power, 2014-06-02 13:21:33]

Change committed as 274222