Replace fixed string uses of AddHeaderFromString

Uses of AddHeaderFromString() with a static string may as well be
replaced with SetHeader(). Do so.

BUG=None

Review-Url: https://codereview.chromium.org/2236933005
Cr-Commit-Position: refs/heads/master@{#418161}
void MockNetworkTransaction::CallbackLater(const CompletionCallback& callback,
                                           int result) {
  base::ThreadTaskRunnerHandle::Get()->PostTask(
      FROM_HERE, base::Bind(&MockNetworkTransaction::RunCallback,
                            weak_factory_.GetWeakPtr(), callback, result));
}

void MockNetworkTransaction::CallbackLater(const CompletionCallback& callback,
                                           int result) {
  base::ThreadTaskRunnerHandle::Get()->PostTask(
      FROM_HERE, base::Bind(&MockNetworkTransaction::RunCallback,
                            weak_factory_.GetWeakPtr(), callback, result));
}
