chakrashim: Add 'Debug' object only when needed

If `--debug` is specified, the in-built 'Debug' object is exposed by
chakra.dll that has some [APIs](https://msdn.microsoft.com/en-us/library/bs12a9wf(v=vs.94).aspx) that node-uwp relies on. With my change in #155,
I had overriden 'Debug' global object and hence certain Debug APIs stopped
working with `--debug` switch. The original intent of adding `Debug` object
was that `util.js` fetches this object in Debugging context. In absense of
`--debug` flag this object is unavailable and hence `util.js` throws TypeError.

The fix is to expose `Debug` object only in the DebugContext (called from
`util.js`). If ran with `--debug`, by the time `utill.js` code executes, engine
would have already expose in-built `Debug` object and we won't overwrite it.
Without `--debug` we would override `Debug` object.

Thanks @agarwal-sandeep for helping debugging this issue.

Fixes : #175

deps/chakrashim/lib/chakra_shim.js

@@ -19,11 +19,6 @@
 // IN THE SOFTWARE.
 'use strict';
 
-// CHAKRA-TODO doesn't implement the debugger. So add a dummy 'Debug' on
-// global object for now.
-Object.defineProperty(this, 'Debug',
-     { value: {}, enumerable: false, configurable: false, writable: false });
-
 (function() {
   // Save original builtIns
   var

deps/chakrashim/src/v8debug.cc

@@ -63,6 +63,23 @@ Local<Context> Debug::GetDebugContext() {
     HandleScope scope(isolate);
     g_debugContext = *Context::New(isolate);
     JsAddRef(g_debugContext, nullptr);
+
+    // CHAKRA-TODO: Chakra doesn't fully implement the debugger without
+    // --debug flag. Add a dummy 'Debug' on global object if it doesn't
+    // already exist.
+    {
+      Context::Scope context_scope(g_debugContext);
+      wchar_t * debugScript = L""
+        "if(this['Debug'] == undefined) { "
+           "Object.defineProperty(this, 'Debug', { value: {}, "
+                "enumerable : false, configurable : false, writable : false "
+            "}); "
+         "}";
+      JsValueRef result;
+      if (JsRunScript(debugScript, JS_SOURCE_CONTEXT_NONE, L"", &result) != JsNoError) {
+        return Local<Context>();
+      }
+    }
   }
 
   return static_cast<Context*>(g_debugContext);

test/parallel/test-cli-syntax.js

@@ -55,9 +55,9 @@ var syntaxArgs = [
 
     // stderr should have a syntax error message
     var match = c.stderr.match(common.engineSpecificMessage({
-                                v8: /^SyntaxError: Unexpected identifier$/m,
-                                chakracore: /^SyntaxError: Expected ';'$/m})
-								);
+      v8: /^SyntaxError: Unexpected identifier$/m,
+      chakracore: /^SyntaxError: Expected ';'$/m})
+	);
     assert(match, 'stderr incorrect');
 
     assert.equal(c.status, 1, 'code == ' + c.status);