Add read barriers for the GC roots in Instrumentation.

Bug: 12687968
Change-Id: I324e2f950ce4500b0e00722044af3a9c82487b23
diff --git a/runtime/debugger.cc b/runtime/debugger.cc
index 4cf4c09..bebc0c0 100644
--- a/runtime/debugger.cc
+++ b/runtime/debugger.cc
@@ -3048,7 +3048,7 @@
 
 // Sanity checks all existing breakpoints on the same method.
 static void SanityCheckExistingBreakpoints(mirror::ArtMethod* m, bool need_full_deoptimization)
-    EXCLUSIVE_LOCKS_REQUIRED(Locks::breakpoint_lock_)  {
+    EXCLUSIVE_LOCKS_REQUIRED(Locks::breakpoint_lock_) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
   if (kIsDebugBuild) {
     for (const Breakpoint& breakpoint : gBreakpoints) {
       CHECK_EQ(need_full_deoptimization, breakpoint.NeedFullDeoptimization());