Mark more roots.

This is most of the stuff. (Everything that currently exists, though there's
more to come.)

Change-Id: I235a21b006820a027c494374a5b52ffefed89c32
diff --git a/src/thread.h b/src/thread.h
index ab0660e..ae07fa6 100644
--- a/src/thread.h
+++ b/src/thread.h
@@ -406,6 +406,8 @@
   // Allocate stack trace
   ObjectArray<StackTraceElement>* AllocStackTrace();
 
+  void VisitRoots(Heap::RootVisitor* visitor, void* arg) const;
+
  private:
   Thread()
       : id_(1234),
@@ -509,6 +511,8 @@
     lock_->Unlock();
   };
 
+  void VisitRoots(Heap::RootVisitor* visitor, void* arg) const;
+
  private:
   ThreadList();