Bug 1423000 - Count jemalloc_stats as an operation. r=njn

While jemalloc_stats is not actively doing anything, it can be
cumbersome to not have it count as an operation, because the operation
count shown on jemalloc_stats doesn't match the line number in the input
replay log, and the offset grows as the number of jemalloc_stats
operations grows.

While here, also update a comment about the replay log format.

--HG--
extra : rebase_source : da0ad9a990487ebdfadae7f8fcfad85e82b482fc

memory/replace/logalloc/replay/Replay.cpp

@@ -421,6 +421,7 @@ public:
     if (aArgs) {
       die("Malformed input");
     }
+    mOps++;
     jemalloc_stats_t stats;
     ::jemalloc_stats(&stats);
     FdPrintf(mStdErr,
@@ -448,7 +449,7 @@ main()
 
   /* Read log from stdin and dispatch function calls to the Replay instance.
    * The log format is essentially:
-   *   <pid> <function>([<args>])[=<result>]
+   *   <pid> <tid> <function>([<args>])[=<result>]
    * <args> is a comma separated list of arguments.
    *
    * The logs are expected to be preprocessed so that allocations are