ART: More cutouts for unstarted runtime

Dex objects keep byte buffers to mapped dex files. For annotation
access we create them when compiling, but they must be cleared before
writing an image.

Add some Memory functions.

Bug: 19542228
Change-Id: I995791b43f2b09192d88645821d9e55b45f1b606
diff --git a/runtime/mirror/dex_cache.h b/runtime/mirror/dex_cache.h
index 3c947ab..c548c03 100644
--- a/runtime/mirror/dex_cache.h
+++ b/runtime/mirror/dex_cache.h
@@ -59,6 +59,10 @@
     return GetFieldObject<String>(OFFSET_OF_OBJECT_MEMBER(DexCache, location_));
   }
 
+  static MemberOffset DexOffset() {
+    return OFFSET_OF_OBJECT_MEMBER(DexCache, dex_);
+  }
+
   static MemberOffset StringsOffset() {
     return OFFSET_OF_OBJECT_MEMBER(DexCache, strings_);
   }