Move default Hermes GC config from ReactAndroid to ReactCommon

We want to use the same GC config on iOS. We could copy it in, but I figured it would be better to move it to a shared place that can be accessed on both iOS and Android.

ReactAndroid/src/main/java/com/facebook/hermes/reactexecutor/OnLoad.cpp

@@ -6,6 +6,7 @@
  */
 
 #include <../instrumentation/HermesMemoryDumper.h>
+#include <DefaultHermesGCConfigBuilder-inl.h>
 #include <HermesExecutorFactory.h>
 #include <android/log.h>
 #include <fbjni/fbjni.h>
@@ -31,22 +32,8 @@ static void hermesFatalHandler(const std::string &reason) {
 static std::once_flag flag;
 
 static ::hermes::vm::RuntimeConfig makeRuntimeConfig(jlong heapSizeMB) {
-  namespace vm = ::hermes::vm;
-  auto gcConfigBuilder =
-      vm::GCConfig::Builder()
-          .withName("RN")
-          // For the next two arguments: avoid GC before TTI by initializing the
-          // runtime to allocate directly in the old generation, but revert to
-          // normal operation when we reach the (first) TTI point.
-          .withAllocInYoung(false)
-          .withRevertToYGAtTTI(true);
-
-  if (heapSizeMB > 0) {
-    gcConfigBuilder.withMaxHeapSize(heapSizeMB << 20);
-  }
-
-  return vm::RuntimeConfig::Builder()
-      .withGCConfig(gcConfigBuilder.build())
+  return ::hermes::vm::RuntimeConfig::Builder()
+      .withGCConfig(defaultHermesGCConfigBuilder(heapSizeMB).build())
       .build();
 }
 

ReactCommon/hermes/executor/DefaultHermesGCConfigBuilder-inl.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <cstdint>
+#include <hermes/Public/GCConfig.h>
+#include <hermes/Public/RuntimeConfig.h>
+
+namespace facebook {
+namespace react {
+
+::hermes::vm::GCConfig::Builder defaultHermesGCConfigBuilder(
+  int64_t heapSizeMB
+) {
+  namespace vm = ::hermes::vm;
+  auto gcConfigBuilder =
+      vm::GCConfig::Builder()
+          .withName("RN")
+          // For the next two arguments: avoid GC before TTI by initializing the
+          // runtime to allocate directly in the old generation, but revert to
+          // normal operation when we reach the (first) TTI point.
+          .withAllocInYoung(false)
+          .withRevertToYGAtTTI(true);
+  if (heapSizeMB > 0) {
+    gcConfigBuilder.withMaxHeapSize(heapSizeMB << 20);
+  }
+  return gcConfigBuilder;
+}
+
+} // namespace react
+} // namespace facebook