[debugger] flood function for stepping before calling it.

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index 3830d8eed863c3092529de7f64016e80296a98ca..22676eb37cfdb086c762b141e3cbd2311716eeb3 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -731,6 +731,13 @@ void Debug::ClearAllBreakPoints() {
 
 void Debug::FloodWithOneShot(Handle<JSFunction> function,
                              BreakLocatorType type) {
+  if (!function->shared()->IsSubjectToDebugging()) {
+    // Builtin functions are not subject to stepping, but need to be
+    // deoptimized, because optimized code does not check for debug
+    // step in at call sites.
+    Deoptimizer::DeoptimizeFunction(*function);
+    return;
+  }
   // Make sure the function is compiled and has set up the debug info.
   Handle<SharedFunctionInfo> shared(function->shared());
   if (!EnsureDebugInfo(shared, function)) {
@@ -2279,6 +2286,9 @@ void Debug::HandleDebugBreak() {
 
   isolate_->stack_guard()->ClearDebugBreak();
 
+  // Clear stepping to avoid duplicate breaks.
+  ClearStepping();
+
   ProcessDebugMessages(debug_command_only);
 }