Force inlining on trivial accessors.

Make volatility for GetFieldObject a template parameter.
Move some trivial mirror::String routines to a -inl.h.

Bug: 14285442

Change-Id: Ie23b11d4f18cb15a62c3bbb42837a8aaf6b68f92
diff --git a/runtime/mirror/object_reference.h b/runtime/mirror/object_reference.h
index 72f281d..b63d13d 100644
--- a/runtime/mirror/object_reference.h
+++ b/runtime/mirror/object_reference.h
@@ -17,7 +17,7 @@
 #ifndef ART_RUNTIME_MIRROR_OBJECT_REFERENCE_H_
 #define ART_RUNTIME_MIRROR_OBJECT_REFERENCE_H_
 
-#include "base/mutex.h"
+#include "base/mutex.h"  // For Locks::mutator_lock_.
 #include "globals.h"
 
 namespace art {