Sanitize headers in Proxy Authentication Required responses

BUG=431504

Review URL: https://codereview.chromium.org/769043003

Cr-Commit-Position: refs/heads/master@{#310014}
size_t SpdyProxyClientSocket::PopulateUserReadBuffer(char* data, size_t len) {
  return read_buffer_queue_.Dequeue(data, len);
}

size_t SpdyProxyClientSocket::PopulateUserReadBuffer(char* data, size_t len) {
  return read_buffer_queue_.Dequeue(data, len);
}
