Adding set_uid to Chapter Class

Adding set_uid function to the Chapter class. It can be used to
achieve deterministic output from the muxer. For example, for files
with only one segment, track number or track id can be used for
chapter uid.

Change-Id: I2e94c6150e32cb9019a6623af7919acc099aa20d

mkvmuxer.hpp

@@ -531,6 +531,10 @@ class Chapter {
                 uint64 start_time_ns,
                 uint64 end_time_ns);
 
+  // Sets the uid for this chapter. Primarily used to enable
+  // deterministic output from the muxer.
+  void set_uid(const uint64 uid) { uid_ = uid; }
+
   // Add a title string to this chapter, per the semantics described
   // here:
   //  http://www.matroska.org/technical/specs/index.html