In components/net/http_loader.rs::http_fetch we need to set secure_connection_start (if we're using a persistent connection). Otherwise, we can probably use components/net/connector.rs for this

On getting, the secureConnectionStart attribute MUST return as follows:

1. The same value as fetchStart, if a persistent connection [RFC7230] is used or the resource is retrieved from relevant application caches or local resources.
2. The time immediately before the user agent starts the handshake process to secure the current connection, if a secure transport is used and the last non-redirected fetch of the resource passes the timing allow check algorithm.
3. zero, otherwise.

Spec: https://w3c.github.io/resource-timing/#dom-performanceresourcetiming-secureconnectionstart