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}
bool MockNetworkTransaction::GetRemoteEndpoint(IPEndPoint* endpoint) const {
  *endpoint = IPEndPoint(IPAddress(127, 0, 0, 1), 80);
  return true;
}

bool MockNetworkTransaction::GetRemoteEndpoint(IPEndPoint* endpoint) const {
  *endpoint = IPEndPoint(IPAddress(127, 0, 0, 1), 80);
  return true;
}
