Make OperationID usable for error handling.

Make OperationID usable for error handling.

Currently, a callback can be called before the FileSystemOperationRunner's
method returns OperationID. So, it is difficult to use OperationID inside the
callback. This CL makes two changes:
- Methods start to return valid OperationID even if the running operation fails.
- Callbacks will be invoked at least in the next message loop.
Then, the caller of FileSystemOperationRunner's methods can know OperationID
before the callback is invoked, and use it in callbacks.

BUG=279288
TEST=Ran unit_tests content_unittests

[hidehiko, 2013-09-02 12:00:11]

Thank you for your review in advance,
- hidehiko

[kinuko, 2013-09-03 00:58:21]

lgtm

[commit-bot: I haz the power, 2013-09-03 01:01:57]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/hidehiko@chromium.org/23477026/1

[commit-bot: I haz the power, 2013-09-03 03:37:22]

Retried try job too often on linux_chromeos for step(s) browser_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_chro...

[hidehiko, 2013-09-06 06:55:13]

On 2013/09/03 03:37:22, I haz the power (commit-bot) wrote:
> Retried try job too often on linux_chromeos for step(s) browser_tests
>
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_chro...

Closing as r221384 is landed.
Thank you for your work, Kinuko!

- hidehiko