Add set_uid() to mkvmuxer::Track.

Allow users of libwebm's MKV muxer to control the Track UID. This,
for example, allows creation of tests that rely on checksum comparisons
to verify output from the library.

Change-Id: I8b052e6dd5af734d76122b2a1b6b16382ffba214

mkvmuxer.hpp

@@ -317,6 +317,7 @@ class Track {
   uint64 number() const { return number_; }
   void set_type(uint64 type) { type_ = type; }
   uint64 type() const { return type_; }
+  void set_uid(uint64 uid) { uid_ = uid; }
   uint64 uid() const { return uid_; }
 
   uint64 codec_private_length() const { return codec_private_length_; }