Move QuicBufferAllocator into its own .cc/.h. No behavior change.

net/quic/core/quic_protocol.cc

Index: net/quic/core/quic_protocol.cc
diff --git a/net/quic/core/quic_protocol.cc b/net/quic/core/quic_protocol.cc
index d9ca3eba0031a20254b23c15bce961a98b8d29be..8db8d57624485030550fa264795bc8304cb3b0eb 100644
--- a/net/quic/core/quic_protocol.cc
+++ b/net/quic/core/quic_protocol.cc
@@ -97,8 +97,6 @@ QuicPublicResetPacket::QuicPublicResetPacket(
     const QuicPacketPublicHeader& header)
     : public_header(header), nonce_proof(0), rejected_packet_number(0) {}
 
-QuicBufferAllocator::~QuicBufferAllocator() = default;
-
 void StreamBufferDeleter::operator()(char* buffer) const {
   if (allocator_ != nullptr && buffer != nullptr) {
     allocator_->Delete(buffer);