Avoid direct access to length_.

Should use accessors for fields of any Java heap object. Interestingly
in this case, as the objects are packed GCC for ARM implements the word
load as 4 byte loads, so this code is more optimal.

Change-Id: Iafd3d8ee9f41fb19b3a68b3e90ad51a8acd3d61b
diff --git a/src/object.h b/src/object.h
index efd456e..63228bc 100644
--- a/src/object.h
+++ b/src/object.h
@@ -1110,7 +1110,7 @@
  protected:
   bool IsValidIndex(int32_t index) const
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-    if (UNLIKELY(index < 0 || index >= length_)) {
+    if (UNLIKELY(index < 0 || index >= GetLength())) {
       return ThrowArrayIndexOutOfBoundsException(index);
     }
     return true;