(Experimental) Add Brooks pointers.

This feature is disabled by default.

Verified that the Brooks pointers are installed correctly by using the
CMS/SS collectors.

Change-Id: Ia9be9814ab6e29169ac85edc4792ce8c81d552a9
diff --git a/runtime/globals.h b/runtime/globals.h
index 8c3ae56..83e3028 100644
--- a/runtime/globals.h
+++ b/runtime/globals.h
@@ -19,6 +19,7 @@
 
 #include <stddef.h>
 #include <stdint.h>
+#include "brooks_pointer.h"
 
 namespace art {
 
@@ -92,6 +93,12 @@
 // code, if possible.
 static constexpr bool kEmbedClassInCode = true;
 
+#ifdef USE_BROOKS_POINTER
+static constexpr bool kUseBrooksPointer = true;
+#else
+static constexpr bool kUseBrooksPointer = false;
+#endif
+
 }  // namespace art
 
 #endif  // ART_RUNTIME_GLOBALS_H_