VC++ - More header guards

See if I can reduce the number of errors in https://ci.appveyor.com/project/cefsharp/cefsharp/builds/21490716

Author: amaitland

CefSharp.Core/Internals/CefRequestContextHandlerAdapter.h

@@ -2,6 +2,9 @@
 //
 // Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
 
+#ifndef CEFSHARP_CORE_INTERNALS_CEFREQUESTCONTEXTHANDLERADAPTER_H_
+#define CEFSHARP_CORE_INTERNALS_CEFREQUESTCONTEXTHANDLERADAPTER_H_
+
 #pragma once
 
 #include "Stdafx.h"
@@ -42,4 +45,5 @@ namespace CefSharp
             IMPLEMENT_REFCOUNTING(CefRequestContextHandlerAdapter);
         };
     }
-}
+}
+#endif  // CEFSHARP_CORE_INTERNALS_CEFREQUESTCONTEXTHANDLERADAPTER_H_

CefSharp.Core/ManagedCefBrowserAdapter.h

@@ -2,6 +2,9 @@
 //
 // Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
 
+#ifndef CEFSHARP_CORE_MANAGEDCEFBROWSERADAPTER_H_
+#define CEFSHARP_CORE_MANAGEDCEFBROWSERADAPTER_H_
+
 #pragma once
 
 #include "Stdafx.h"
@@ -135,3 +138,4 @@ namespace CefSharp
         }
     };
 }
+#endif  // CEFSHARP_CORE_INTERNALS_CLIENTADAPTER_H_