Most likeky it isnt thay the heart stops or starts in response to freezing. The heartbeat signal probably runs 24/7 unless inhibited by freezing. So as soon as it thaws, the signaling bit resumes its pulsing and the rest of the heart begins to beats as it thaws. Remeber too that at this scale frogs can absorb much of thier needed O2 through their skin. A steady heartbeat isnt as essential as it is with us.