[inspector] migrate stepping related methods to debug-interface

src/debug/debug-interface.h

Index: src/debug/debug-interface.h
diff --git a/src/debug/debug-interface.h b/src/debug/debug-interface.h
index e6d178095868a293cae1a1390067b158e8280c3f..0577ecc4ca2b5980aaa979da0e74831b9a2bea47 100644
--- a/src/debug/debug-interface.h
+++ b/src/debug/debug-interface.h
@@ -127,6 +127,17 @@ class DebugInterface {
    */
   static void ChangeBreakOnException(Isolate* isolate,
                                      ExceptionBreakState state);
+
+  enum StepAction {
+    StepOut = 0,   // Step out of the current function.
+    StepNext = 1,  // Step to the next statement in the current function.
+    StepIn = 2,    // Step into new functions invoked or the next statement
+                   // in the current function.
+    StepFrame = 3  // Step into a new frame or return to previous frame.
+  };
+
+  static void PrepareStep(Isolate* isolate, StepAction action);
+  static void ClearStepping(Isolate* isolate);
 };
 
 }  // namespace v8