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/reference_table.h b/runtime/reference_table.h
index 1cd0999..8765442 100644
--- a/runtime/reference_table.h
+++ b/runtime/reference_table.h
@@ -23,6 +23,7 @@
 #include <vector>
 
 #include "base/mutex.h"
+#include "gc_root.h"
 #include "object_callbacks.h"
 
 namespace art {
@@ -50,7 +51,7 @@
   void VisitRoots(RootCallback* visitor, void* arg, uint32_t tid, RootType root_type);
 
  private:
-  typedef std::vector<mirror::Object*> Table;
+  typedef std::vector<GcRoot<mirror::Object>> Table;
   static void Dump(std::ostream& os, Table& entries)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
   friend class IndirectReferenceTable;  // For Dump.