Convert ByteBufferWriter to be type uint8_t

and make follow-on changes.

Bug: webrtc:15665
Change-Id: Ice646f88ba5a09d6a8d9ce70415d8a14d7050d3c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/329781
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41393}

rtc_base/byte_buffer_unittest.cc

diff --git a/rtc_base/byte_buffer_unittest.cc b/rtc_base/byte_buffer_unittest.cc
index 7d38d80..49e2e56 100644
--- a/rtc_base/byte_buffer_unittest.cc
+++ b/rtc_base/byte_buffer_unittest.cc
@@ -166,7 +166,7 @@
   buffer.Clear();
 
   // Write and read bytes
-  char write_bytes[] = "foo";
+  uint8_t write_bytes[] = "foo";
   buffer.WriteBytes(write_bytes, 3);
   ByteBufferReader read_buf7(buffer);
   uint8_t read_bytes[3];
@@ -178,7 +178,7 @@
   buffer.Clear();
 
   // Write and read reserved buffer space
-  char* write_dst = buffer.ReserveWriteBuffer(3);
+  uint8_t* write_dst = buffer.ReserveWriteBuffer(3);
   memcpy(write_dst, write_bytes, 3);
   ByteBufferReader read_buf8(buffer);
   memset(read_bytes, 0, 3);