Merge pull request #1808 from jcanizales/3.0.0-beta-4

Cherrypick: Use public methods to fetch oneofs in generated code.

objectivec/google/protobuf/Struct.pbobjc.m

@@ -237,7 +237,7 @@ void SetGPBValue_NullValue_RawValue(GPBValue *message, int32_t value) {
 
 void GPBValue_ClearKindOneOfCase(GPBValue *message) {
   GPBDescriptor *descriptor = [message descriptor];
-  GPBOneofDescriptor *oneof = descriptor->oneofs_[0];
+  GPBOneofDescriptor *oneof = [descriptor.oneofs objectAtIndex:0];
   GPBMaybeClearOneof(message, oneof, -1, 0);
 }
 #pragma mark - GPBListValue

src/google/protobuf/compiler/objectivec/objectivec_oneof.cc

@@ -119,7 +119,7 @@ void OneofGenerator::GenerateClearFunctionImplementation(io::Printer* printer) {
       variables_,
       "void $owning_message_class$_Clear$capitalized_name$OneOfCase($owning_message_class$ *message) {\n"
       "  GPBDescriptor *descriptor = [message descriptor];\n"
-      "  GPBOneofDescriptor *oneof = descriptor->oneofs_[$raw_index$];\n"
+      "  GPBOneofDescriptor *oneof = [descriptor.oneofs objectAtIndex:$raw_index$];\n"
       "  GPBMaybeClearOneof(message, oneof, $index$, 0);\n"
       "}\n");
 }