InspectorServer: Use the 32-bit data length when sending a request

`length` was inheriting `size_t` type of the `String::length()`, while
everywhere else in the Inspector we expect fixed 32-bit field. On the
architectures where `sizeof(size_t) != sizeof(u32)` this broke the
Inspector communication completely.

Userland/Services/InspectorServer/InspectableProcess.cpp

@@ -73,7 +73,7 @@ String InspectableProcess::wait_for_response()
 void InspectableProcess::send_request(JsonObject const& request)
 {
     auto serialized = request.to_string();
-    auto length = serialized.length();
+    u32 length = serialized.length();
     m_socket->write((u8 const*)&length, sizeof(length));
     m_socket->write(serialized);
 }