[Debugger] Add a ReturnValueScope to correctly handle return values in nested debug breaks.

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index 84a16754e1d8c453dd5930addcefbb15a43df9c2..36e508321725166ba95c94c47daa2c45ca3251c3 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -481,8 +481,6 @@ void Debug::Unload() {
 }
 
 void Debug::Break(JavaScriptFrame* frame) {
-  HandleScope scope(isolate_);
-
   // Initialize LiveEdit.
   LiveEdit::InitializeThreadLocal(this);
 
@@ -2206,7 +2204,6 @@ DebugScope::DebugScope(Debug* debug)
   // Store the previous break id, frame id and return value.
   break_id_ = debug_->break_id();
   break_frame_id_ = debug_->break_frame_id();
-  return_value_ = handle(debug_->return_value(), isolate());
 
   // Create the new break info. If there is no proper frames there is no break
   // frame id.
@@ -2232,11 +2229,18 @@ DebugScope::~DebugScope() {
   // Restore to the previous break state.
   debug_->thread_local_.break_frame_id_ = break_frame_id_;
   debug_->thread_local_.break_id_ = break_id_;
-  debug_->thread_local_.return_value_ = *return_value_;
 
   debug_->UpdateState();
 }
 
+ReturnValueScope::ReturnValueScope(Debug* debug) : debug_(debug) {
+  return_value_ = debug_->return_value_handle();
+}
+
+ReturnValueScope::~ReturnValueScope() {
+  debug_->set_return_value(*return_value_);
+}
+
 bool Debug::PerformSideEffectCheck(Handle<JSFunction> function) {
   DCHECK(isolate_->needs_side_effect_check());
   DisallowJavascriptExecution no_js(isolate_);