Raise initial padding number to 8

Protect initial handshake lengths.

Common client initial sequence:

- TLS: ClientHello
- TLS: ChangeCipherSpec, Finished
- H2: Magic, SETTINGS, WINDOW_UPDATE
- H2: HEADERS: GET
- H2: SETTINGS ACK

Common server initial sequence:

- TLS: ServerHello, ChangeCipherSpec, ...
- TLS: Certificate, ...
- H2: SETTINGS
- H2: WINDOW_UPDATE
- H2: SETTINGS ACK
- H2: HEADERS: 200 OK

src/net/tools/naive/naive_connection.cc

@@ -42,7 +42,7 @@ namespace net {
 
 namespace {
 constexpr int kBufferSize = 64 * 1024;
-constexpr int kFirstPaddings = 4;
+constexpr int kFirstPaddings = 8;
 constexpr int kPaddingHeaderSize = 3;
 constexpr int kMaxPaddingSize = 255;
 }  // namespace