Add source position recording for variable loads

test/mjsunit/debug-backtrace.js

 // Copyright 2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 177 matching lines @@
       assertEquals('undefined', response.lookup(frame.arguments[1].value.ref).type);
 
       json = '{"seq":0,"type":"request","command":"frame","arguments":{"number":1}}'
       response = new ParsedResponse(dcp.processDebugJSONRequest(json));
       frame = response.body();
       assertEquals(1, frame.index);
       assertEquals("", response.lookup(frame.func.ref).name);
       assertEquals("m", response.lookup(frame.func.ref).inferredName);
       assertFalse(frame.constructCall);
       assertEquals(35, frame.line);
-      assertEquals(2, frame.column);
+      assertEquals(6, frame.column);
       assertEquals(0, frame.arguments.length);
 
       json = '{"seq":0,"type":"request","command":"frame","arguments":{"number":3}}'
       response = new ParsedResponse(dcp.processDebugJSONRequest(json));
       frame = response.body();
       assertEquals(3, frame.index);
       assertEquals("", response.lookup(frame.func.ref).name);
 
       // Source slices for the individual frames (they all refer to this script).
       json = '{"seq":0,"type":"request","command":"source",' +
@@ skipping 53 matching lines @@
 // Add the debug event listener.
 Debug.setListener(listener);
 
 // Set a break point and call to invoke the debug event listener.
 Debug.setBreakPoint(f, 0, 0);
 g();
 
 // Make sure that the debug event listener vas invoked.
 assertFalse(exception, "exception in listener");
 assertTrue(listenerCalled);
-