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/common/net/http_header_utils.h

@@ -13,7 +13,7 @@
 
 namespace HttpHeaderUtils {
 
-typedef std::multimap<CefString, CefString> HeaderMap;
+using HeaderMap = std::multimap<CefString, CefString>;
 
 std::string GenerateHeaders(const HeaderMap& map);
 void ParseHeaders(const std::string& header_str, HeaderMap& map);