Mark breakpoint roots

Used to prevent class unloading on methods that have breakpoints.

Bug: 22720414
Change-Id: I9aee8bcbfdf253607e89dfc55a50ba3f11d99206
diff --git a/runtime/debugger.h b/runtime/debugger.h
index b3617e4..e908304 100644
--- a/runtime/debugger.h
+++ b/runtime/debugger.h
@@ -646,6 +646,7 @@
   static void DdmSendChunkV(uint32_t type, const iovec* iov, int iov_count)
       SHARED_REQUIRES(Locks::mutator_lock_);
 
+  // Visit breakpoint roots, used to prevent unloading of methods with breakpoints.
   static void VisitRoots(RootVisitor* visitor)
       SHARED_REQUIRES(Locks::mutator_lock_);