Crash fix in fileapi::FileSystemOperation::DidGetUsageAndQuotaAndRunTask

https://chromiumcodereview.appspot.com/10008047 introduced delete-with-inflight-tasks in Write sequence but I failed to convert this callback to use WeakPtr().

BUG=128178
TEST=manual test

Review URL: https://chromiumcodereview.appspot.com/10408006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@137635 0039d316-1c4b-4281-b951-d872f2087c98
void FileSystemOperation::DoCopy(const StatusCallback& callback) {
  FileSystemFileUtilProxy::Copy(
      &operation_context_,
      src_util_, dest_util_,
      src_path_, dest_path_,
      base::Bind(&FileSystemOperation::DidFinishFileOperation,
                 base::Owned(this), callback));
}

void FileSystemOperation::DoCopy(const StatusCallback& callback) {
  FileSystemFileUtilProxy::Copy(
      &operation_context_,
      src_util_, dest_util_,
      src_path_, dest_path_,
      base::Bind(&FileSystemOperation::DidFinishFileOperation,
                 base::Owned(this), callback));
}
