Move V8 snapshot loading code from isolate_holder to gin/v8_startup_data.{h,cc}.

gin/shell/gin_main.cc

Index: gin/shell/gin_main.cc
diff --git a/gin/shell/gin_main.cc b/gin/shell/gin_main.cc
index 532f996cefdaa477020c196335368680ea3572c0..32a4e8cf94fe3ade9c25fd4ebba2ec709d516b91 100644
--- a/gin/shell/gin_main.cc
+++ b/gin/shell/gin_main.cc
@@ -13,6 +13,7 @@
 #include "gin/modules/module_runner_delegate.h"
 #include "gin/public/isolate_holder.h"
 #include "gin/try_catch.h"
+#include "gin/v8_initializer.h"
 
 namespace gin {
 namespace {
@@ -60,10 +61,10 @@ int main(int argc, char** argv) {
   base::CommandLine::Init(argc, argv);
   base::i18n::InitializeICU();
 #ifdef V8_USE_EXTERNAL_STARTUP_DATA
-  gin::IsolateHolder::LoadV8Snapshot();
+  gin::V8Initializer::LoadV8Snapshot();
 #endif
 
-  gin::IsolateHolder::Initialize(gin::IsolateHolder::kStrictMode,
+  gin::V8Initializer::Initialize(gin::IsolateHolder::kStrictMode,
                                  gin::ArrayBufferAllocator::SharedInstance());
   gin::IsolateHolder instance;