[inspector] migrate stepping related methods to debug-interface

src/inspector/debugger_script_externs.js

Index: src/inspector/debugger_script_externs.js
diff --git a/src/inspector/debugger_script_externs.js b/src/inspector/debugger_script_externs.js
index 88a52360d23e051df41ccba6f7d32603a8f795a4..daaf8c6b24796f01e98c6b71efba50378a63b10b 100644
--- a/src/inspector/debugger_script_externs.js
+++ b/src/inspector/debugger_script_externs.js
@@ -61,8 +61,6 @@ var JavaScriptCallFrame;
  */
 var Debug = {};
 
-Debug.clearStepping = function() {}
-
 Debug.clearAllBreakPoints = function() {}
 
 /** @return {!Array<!Script>} */
@@ -192,9 +190,6 @@ BreakEvent.prototype.breakPointsHit = function() {}
 /** @interface */
 function ExecutionState() {}
 
-/** @param {!Debug.StepAction} action */
-ExecutionState.prototype.prepareStep = function(action) {}
-
 /**
  * @param {string} source
  * @param {boolean} disableBreak