Use Content-Type charset when decoding download filenames (#3135)

[linesight]

Fixes #3135

When a server's Content-Disposition carries non-ASCII filename bytes without an RFC 5987 filename*, HttpContentDisposition::DecodeWord needs a charset hint. CEF currently passes an empty string, so DecodeWord falls back through IsStringUTF8() to SysNativeMBToWide() — the platform locale. On macOS and Linux that produces garbled filenames whenever the server's encoding doesn't match the user's locale, most commonly Chinese sites that emit GBK filenames with Content-Type: ...; charset=gb2312.

The fix pulls the charset off the in-flight response headers and passes it as referrer_charset.

When no charset is declared, GetCharset is a no-op and behavior is identical to before.

[magreenblatt]

Thanks for the fix!

• ChineseFilenameContentTypeCharset: clarify this exercises the charset
  extraction path but would pass even without the fix (UTF-8 auto-detection)

As you mention, the test coverage in this PR doesn't actually cover the fix. The easiest option is to extend the existing CefTestServer API with a new method that takes all header data a single void* buffer.

  A new method on CefTestServerConnection:
  
  virtual void SendHttpResponseWithRawHeaders(const void* header_data,
                                              size_t header_data_size,
                                              const void* response_data,
                                              size_t response_data_size) = 0;

  header_data is the complete raw HTTP response header block including the status line, e.g.:

  HTTP/1.1 200 OK\r\n
  Content-Type: application/vnd.ms-excel; charset=gb2312\r\n
  Content-Disposition: attachment; filename=<raw GBK bytes>\r\n

  response_data is the response body. Both are void* + size_t pairs, which the CEF translator already handles (same pattern as
  the existing data/data_size parameters on SendHttp200Response and SendHttpResponse).

  The implementation in CefTestServerConnectionImpl uses the existing RawHttpResponse class from
  net/test/embedded_test_server/http_response.h, which takes a headers string and contents string and sends them directly to
  the HttpResponseDelegate via SendRawResponseHeaders() — no CefString conversion anywhere. It follows the same thread-bouncing
  pattern as the existing SendBasicHttpResponse private method.

  This enables the GBK download filename test: the test constructs a std::string containing raw GBK bytes in the
  Content-Disposition header and charset=gb2312 in the Content-Type header, passes it as header_data. The bytes reach
  Chromium's network stack intact, GetCharset() returns gb2312, the fix passes it to net::GenerateFileName(), and DecodeWord
  decodes the GBK filename correctly.