Remove DISALLOW_ macro from libcef/ (see issue #3234)

Also perform related C++ cleanup:
- Use =default instead of {} for default implementations of
  constructors/destructors.
- Replace typedef with using.

libcef/browser/gpu/external_texture_manager.h

@@ -36,7 +36,7 @@ class GPU_GLES2_EXPORT ExternalTextureManager {
   void DeleteTexture(void* handle, TextureManager* tex_man);
 
  private:
-  typedef std::map<void*, scoped_refptr<gl::GLImage>> ExternalSurfaceMap;
+  using ExternalSurfaceMap = std::map<void*, scoped_refptr<gl::GLImage>>;
   ExternalSurfaceMap surfaceMap_;
 };