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/platform/api/quic_mem_slice_test.cc b/quic/platform/api/quic_mem_slice_test.cc
index 185f536..d441afa 100644
--- a/quic/platform/api/quic_mem_slice_test.cc
+++ b/quic/platform/api/quic_mem_slice_test.cc
@@ -15,7 +15,7 @@
  public:
   QuicMemSliceTest() {
     size_t length = 1024;
-    slice_ = QuicMemSlice(&allocator_, length);
+    slice_ = QuicMemSlice(MakeUniqueBuffer(&allocator_, length), length);
     orig_data_ = slice_.data();
     orig_length_ = slice_.length();
   }