Add move assignment operator to QuicStreamSequencer

This is required for upcoming QUIC+TLS ALPN extraction.
A new extractor class will be a member to
QuicBufferedPacketStore::BufferedPacketList which needs
a move assignment operator, and it uses QuicStreamSequencer,
therefore we need that to support move assignment. Similarly,
that requires adding move asignment to QuicStreamSequencerBuffer
which also requires making two const members non-const.

gfe-relnote: n/a, added unused operator
PiperOrigin-RevId: 307459373
Change-Id: I3091cb7a6a27a6b01e528417500c18746f3adc3e

quic/core/quic_stream_sequencer.h

diff --git a/quic/core/quic_stream_sequencer.h b/quic/core/quic_stream_sequencer.h
index 50b5b68..a74526b 100644
--- a/quic/core/quic_stream_sequencer.h
+++ b/quic/core/quic_stream_sequencer.h
@@ -50,6 +50,7 @@
   QuicStreamSequencer(const QuicStreamSequencer&) = delete;
   QuicStreamSequencer(QuicStreamSequencer&&) = default;
   QuicStreamSequencer& operator=(const QuicStreamSequencer&) = delete;
+  QuicStreamSequencer& operator=(QuicStreamSequencer&&) = default;
   virtual ~QuicStreamSequencer();
 
   // If the frame is the next one we need in order to process in-order data,

quic/core/quic_stream_sequencer_buffer.h

diff --git a/quic/core/quic_stream_sequencer_buffer.h b/quic/core/quic_stream_sequencer_buffer.h
index 356b62f..0ad9841 100644
--- a/quic/core/quic_stream_sequencer_buffer.h
+++ b/quic/core/quic_stream_sequencer_buffer.h
@@ -94,6 +94,7 @@
   QuicStreamSequencerBuffer(QuicStreamSequencerBuffer&&) = default;
   QuicStreamSequencerBuffer& operator=(const QuicStreamSequencerBuffer&) =
       delete;
+  QuicStreamSequencerBuffer& operator=(QuicStreamSequencerBuffer&&) = default;
   ~QuicStreamSequencerBuffer();
 
   // Free the space used to buffer data.
@@ -212,10 +213,10 @@
   std::string ReceivedFramesDebugString() const;
 
   // The maximum total capacity of this buffer in byte, as constructed.
-  const size_t max_buffer_capacity_bytes_;
+  size_t max_buffer_capacity_bytes_;
 
   // How many blocks this buffer would need when it reaches full capacity.
-  const size_t blocks_count_;
+  size_t blocks_count_;
 
   // Number of bytes read out of buffer.
   QuicStreamOffset total_bytes_read_;