Add a proper API to create memslices from the QUIC code.

Current API provides an (allocator, size) constructor.  However, this just creates a buffer with random data in it and no way to change it, since memslices are read-only.  Current code uses const_cast; this CL creates a proper API and updates all existing call sites to use it.

gfe-relnote: n/a (no functional change)
PiperOrigin-RevId: 287072774
Change-Id: Ie4dd2eae6db2ec91b087f5d41887ea3948ee411a
diff --git a/quic/masque/masque_compression_engine.h b/quic/masque/masque_compression_engine.h
index 32ffc9d..95328df 100644
--- a/quic/masque/masque_compression_engine.h
+++ b/quic/masque/masque_compression_engine.h
@@ -104,7 +104,7 @@
                                     uint8_t first_byte,
                                     bool long_header,
                                     QuicDataReader* reader,
-                                    QuicMemSlice* slice);
+                                    QuicDataWriter* writer);
 
   // Parses compression context from flow ID 0 during decompression.
   bool ParseCompressionContext(QuicDataReader* reader,