Slightly refactor stack map stats printing.

Take advantage of the "operator" overload and move
the pretty print method so it can be better reused.

Test: m dump-oat
Change-Id: I46b169e044f2c4481bb2bc277601d61f873c9950
diff --git a/runtime/stack_map.h b/runtime/stack_map.h
index e8e57aa..ce577c0 100644
--- a/runtime/stack_map.h
+++ b/runtime/stack_map.h
@@ -424,7 +424,7 @@
             InstructionSet instruction_set) const;
 
   // Accumulate code info size statistics into the given Stats tree.
-  static void CollectSizeStats(const uint8_t* code_info, /*out*/ Stats* parent);
+  static void CollectSizeStats(const uint8_t* code_info, /*out*/ Stats& parent);
 
   ALWAYS_INLINE static bool HasInlineInfo(const uint8_t* code_info_data) {
     return (*code_info_data & kHasInlineInfo) != 0;