Fix bug introduced by r21269.

runtime/vm/pages.h

Index: runtime/vm/pages.h
diff --git a/runtime/vm/pages.h b/runtime/vm/pages.h
index 0d4bb02ed29facf87895e4b8e2dff57ea1ed47f8..f89b083db8cf14a58deb24fd7203710fca55f6bb 100644
--- a/runtime/vm/pages.h
+++ b/runtime/vm/pages.h
@@ -35,8 +35,7 @@ class HeapPage {
   }
 
   uword object_start() const {
-    return (reinterpret_cast<uword>(this) +
-            Utils::RoundUp(sizeof(HeapPage), OS::kMaxPreferredCodeAlignment));
+    return (reinterpret_cast<uword>(this) + ObjectStartOffset());
   }
   uword object_end() const {
     return object_end_;
@@ -59,6 +58,10 @@ class HeapPage {
 
   void WriteProtect(bool read_only);
 
+  static intptr_t ObjectStartOffset() {
+    return Utils::RoundUp(sizeof(HeapPage), OS::kMaxPreferredCodeAlignment);
+  }
+
  private:
   void set_object_end(uword val) {
     ASSERT((val & kObjectAlignmentMask) == kOldObjectAlignmentOffset);