Implement the DDMS heap walking (for native and managed heaps).

This gets you the DDMS histograms of what's on your heaps.

Change-Id: I7133d044030b10a787277faf3a77e20c565e69c5
diff --git a/src/space.h b/src/space.h
index 84d679a..eff0da5 100644
--- a/src/space.h
+++ b/src/space.h
@@ -97,6 +97,8 @@
     growth_limit_ = limit_;
   }
 
+  void Walk(void(*callback)(const void*, size_t, const void*, size_t, void*), void* arg);
+
  private:
   // The boundary tag overhead.
   static const size_t kChunkOverhead = kWordSize;