Change nested_root accessor to be const function.

The `<field>_nested_root()` is not viable from const object. (We usually get `const Monster *`.)

Change-Id: I0d0adcb38dd974318608417ee3094c34fb9c480d

src/idl_gen_cpp.cpp

@@ -242,7 +242,7 @@ static void GenTable(const Parser &parser, StructDef &struct_def,
         auto nested_root = parser.structs_.Lookup(nested->constant);
         assert(nested_root);  // Guaranteed to exist by parser.
         code += "  const " + nested_root->name + " *" + field.name;
-        code += "_nested_root() { return flatbuffers::GetRoot<";
+        code += "_nested_root() const { return flatbuffers::GetRoot<";
         code += nested_root->name + ">(" + field.name + "()->Data()); }\n";
       }
       // Generate a comparison function for this field if it is a key.

tests/monster_test_generated.h

@@ -138,7 +138,7 @@ struct Monster FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   const flatbuffers::Vector<flatbuffers::Offset<Monster>> *testarrayoftables() const { return GetPointer<const flatbuffers::Vector<flatbuffers::Offset<Monster>> *>(26); }
   const Monster *enemy() const { return GetPointer<const Monster *>(28); }
   const flatbuffers::Vector<uint8_t> *testnestedflatbuffer() const { return GetPointer<const flatbuffers::Vector<uint8_t> *>(30); }
-  const Monster *testnestedflatbuffer_nested_root() { return flatbuffers::GetRoot<Monster>(testnestedflatbuffer()->Data()); }
+  const Monster *testnestedflatbuffer_nested_root() const { return flatbuffers::GetRoot<Monster>(testnestedflatbuffer()->Data()); }
   const Stat *testempty() const { return GetPointer<const Stat *>(32); }
   uint8_t testbool() const { return GetField<uint8_t>(34, 0); }
   bool Verify(flatbuffers::Verifier &verifier) const {