Split out libplatform into a separate libary

src/libplatform/default-platform.cc

Index: src/libplatform/default-platform.cc
diff --git a/src/libplatform/default-platform.cc b/src/libplatform/default-platform.cc
index 2d346825722d8acd654c79d80a83c0ac5ccb820e..9a2c30dcdefa769913205aac21ab68352efaf8e9 100644
--- a/src/libplatform/default-platform.cc
+++ b/src/libplatform/default-platform.cc
@@ -15,6 +15,14 @@ namespace v8 {
 namespace platform {
 
 
+v8::Platform* CreateDefaultPlatform(int thread_pool_size) {
+  DefaultPlatform* platform = new DefaultPlatform();
+  platform->SetThreadPoolSize(thread_pool_size);
+  platform->EnsureInitialized();
+  return platform;
+}
+
+
 const int DefaultPlatform::kMaxThreadPoolSize = 4;