Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/runtime/debugger.cc b/runtime/debugger.cc
index fe726aa..5a31c87 100644
--- a/runtime/debugger.cc
+++ b/runtime/debugger.cc
@@ -1814,7 +1814,7 @@
 static int GetStackDepth(Thread* thread)
     SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
   struct CountStackDepthVisitor : public StackVisitor {
-    CountStackDepthVisitor(Thread* thread)
+    explicit CountStackDepthVisitor(Thread* thread)
         : StackVisitor(thread, NULL), depth(0) {}
 
     // TODO: Enable annotalysis. We know lock is held in constructor, but abstraction confuses
@@ -2458,7 +2458,7 @@
   //
 
   struct SingleStepStackVisitor : public StackVisitor {
-    SingleStepStackVisitor(Thread* thread)
+    explicit SingleStepStackVisitor(Thread* thread)
         EXCLUSIVE_LOCKS_REQUIRED(Locks::breakpoint_lock_)
         SHARED_LOCKS_REQUIRED(Locks::mutator_lock_)
         : StackVisitor(thread, NULL) {