DevTools: fixed scheduleStepInto for Node 8.x

Node 8.x does not support breakOnAsyncCall param of
Debugger.stepInto method.

R=dgozman@chromium.org

Bug: none
Change-Id: I0e59e0a3742b85a2fd466ac1bac5f7b75aa7f841
Reviewed-on: https://chromium-review.googlesource.com/c/1352533Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612075}

third_party/blink/renderer/devtools/front_end/sdk/DebuggerModel.js

@@ -228,6 +228,9 @@ SDK.DebuggerModel = class extends SDK.SDKModel {
   }
 
   scheduleStepIntoAsync() {
+    // Node v8.x does not support breakOnAsyncCall flag but supports old style schdeuleStepIntoAsync.
+    // End-of-life of Node 8.x is around December 2019.
+    this._agent.scheduleStepIntoAsync();
     this._agent.invoke_stepInto({breakOnAsyncCall: true});
   }