Revert 121620 - Refactor FileSystemOperation to take callback for each method.

Revert 121620 - Refactor FileSystemOperation to take callback for each method.

This patch is the first step for supporting cross-filesystem copy/move on
the Filesystem API implementation. To accomplish it, I'm planning to
crack FileSystemOperation::{Move,Copy} to a series of other FSO operations.
For it, per-method callback is more handy.

BUG=110121
TEST=*File*


Review URL: http://codereview.chromium.org/9372044

TBR=kinaba@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=121623