Deduplicate the code that hardcodes the array header layout.

Get rid of HeaderSize() in array-inl.h and use DataOffset() instead.

Bug: 12687968
Change-Id: Ic81cf3fa6bb9b2440d351a73f5fd6a2d6908d15b
diff --git a/runtime/offsets.h b/runtime/offsets.h
index e2dba9d..ed4e49e 100644
--- a/runtime/offsets.h
+++ b/runtime/offsets.h
@@ -32,6 +32,10 @@
   uint32_t Uint32Value() const {
     return static_cast<uint32_t>(val_);
   }
+  size_t SizeValue() const {
+    return val_;
+  }
+
  protected:
   size_t val_;
 };