src: enable context snapshot after running per-context scripts

At build time, snapshot the context after running per-context scripts
in the main instance, and in the final build, deserialize the
context in the main instance.

This provides a ~5% in the misc/startup benchmark when the instance
is launched within a process that runs test/fixtures/semicolon.js.

src/node_main_instance.cc

@@ -12,6 +12,8 @@ using v8::Local;
 using v8::Locker;
 using v8::SealHandleScope;
 
+const size_t NodeMainInstance::kNodeContextIndex = 0;
+
 NodeMainInstance::NodeMainInstance(Isolate* isolate,
                                    uv_loop_t* event_loop,
                                    MultiIsolatePlatform* platform,
@@ -171,9 +173,13 @@ std::unique_ptr<Environment> NodeMainInstance::CreateMainEnvironment(
     isolate_->GetHeapProfiler()->StartTrackingHeapObjects(true);
   }
 
-  Local<Context> context = NewContext(isolate_);
+  Local<Context> context;
   if (deserialize_mode_) {
+    context =
+        Context::FromSnapshot(isolate_, kNodeContextIndex).ToLocalChecked();
     SetIsolateUpForNode(isolate_, IsolateSettingCategories::kErrorHandlers);
+  } else {
+    context = NewContext(isolate_);
   }
 
   CHECK(!context.IsEmpty());

src/node_main_instance.h

@@ -75,6 +75,8 @@ class NodeMainInstance {
   static const IndexArray* GetIsolateDataIndexes();
   static v8::StartupData* GetEmbeddedSnapshotBlob();
 
+  static const size_t kNodeContextIndex;
+
  private:
   NodeMainInstance(const NodeMainInstance&) = delete;
   NodeMainInstance& operator=(const NodeMainInstance&) = delete;

tools/snapshot/snapshot_builder.cc

@@ -105,6 +105,9 @@ std::string SnapshotBuilder::Generate(
       HandleScope scope(isolate);
       creator.SetDefaultContext(Context::New(isolate));
       isolate_data_indexes = main_instance->isolate_data()->Serialize(&creator);
+
+      size_t index = creator.AddContext(NewContext(isolate));
+      CHECK_EQ(index, NodeMainInstance::kNodeContextIndex);
     }
 
     // Must be out of HandleScope