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/common_runtime_test.cc b/runtime/common_runtime_test.cc
index a43a645..8e363c4 100644
--- a/runtime/common_runtime_test.cc
+++ b/runtime/common_runtime_test.cc
@@ -30,6 +30,7 @@
 #include "class_linker.h"
 #include "compiler_callbacks.h"
 #include "dex_file.h"
+#include "gc_root-inl.h"
 #include "gc/heap.h"
 #include "gtest/gtest.h"
 #include "jni_internal.h"