isHttp2Available() only inspected "resource" timing entries, which can be
empty right after a fresh login or session restore, before any subresource
finishes loading. On an HTTP/2 page this made getMaxAccounts() fall back to
the HTTP/1.1 cap of five accounts. Include the "navigation" timing entry,
whose nextHopProtocol is known immediately, so h2/h3 is recognized on first
load. Environments without timing data still return false (no h2 assumed).
