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::DidFileExists(
    const StatusCallback& callback,
    base::PlatformFileError rv,
    const base::PlatformFileInfo& file_info,
    const FilePath& unused) {
  if (rv == base::PLATFORM_FILE_OK && file_info.is_directory)
    rv = base::PLATFORM_FILE_ERROR_NOT_A_FILE;
  callback.Run(rv);
}

void FileSystemOperation::DidFileExists(
    const StatusCallback& callback,
    base::PlatformFileError rv,
    const base::PlatformFileInfo& file_info,
    const FilePath& unused) {
  if (rv == base::PLATFORM_FILE_OK && file_info.is_directory)
    rv = base::PLATFORM_FILE_ERROR_NOT_A_FILE;
  callback.Run(rv);
}
