LogDog: Properly emit Butler stream magic.

The LogDog Butler protocol was not properly implemented: it was missing
the magic byte sequence that must precede the exchange. Add that
sequence to the stream implementation.

BUG=chromium:628770
TEST=local
  - Ran with live Butler instance, observed successful stream
    negotiation.

Review-Url: https://codereview.chromium.org/2251413004
NOKEYCHECK=True
GitOrigin-RevId: 999aff1abdcaf37cfed37f7451ea0a2f54a7fdea