Add GcRoot to clean up and enforce read barriers.

Introduce a value-type wrapper around Object* for GC roots so that 1)
we won't have to directly add the read barrier code in many places and
2) we can avoid accidentally bypassing/missing read barriers on GC
roots (the GcRoot interface ensures that the read barrier is executed
on a read).

The jdwp test passed.

Bug: 12687968
Change-Id: Ib167c7c325b3c7e3900133578815f04d219972a1
diff --git a/runtime/monitor.h b/runtime/monitor.h
index 0d0ad0b..26d43c9 100644
--- a/runtime/monitor.h
+++ b/runtime/monitor.h
@@ -26,8 +26,9 @@
 
 #include "atomic.h"
 #include "base/mutex.h"
+#include "gc_root.h"
 #include "object_callbacks.h"
-#include "read_barrier.h"
+#include "read_barrier_option.h"
 #include "thread_state.h"
 
 namespace art {
@@ -95,7 +96,7 @@
 
   template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
   mirror::Object* GetObject() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-    return ReadBarrier::BarrierForRoot<mirror::Object, kReadBarrierOption>(&obj_);
+    return obj_.Read<kReadBarrierOption>();
   }
 
   void SetObject(mirror::Object* object);
@@ -197,7 +198,7 @@
   // What object are we part of. This is a weak root. Do not access
   // this directly, use GetObject() to read it so it will be guarded
   // by a read barrier.
-  mirror::Object* obj_;
+  GcRoot<mirror::Object> obj_;
 
   // Threads currently waiting on this monitor.
   Thread* wait_set_ GUARDED_BY(monitor_lock_);