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

gin/shell_runner_unittest.cc

Index: gin/shell_runner_unittest.cc
diff --git a/gin/shell_runner_unittest.cc b/gin/shell_runner_unittest.cc
index 7134de16dc72ecc94fe96541b3182832904642ec..f5100cbcf6b145d297d1942bddf46b8ee64d1f44 100644
--- a/gin/shell_runner_unittest.cc
+++ b/gin/shell_runner_unittest.cc
@@ -8,6 +8,7 @@
 #include "gin/array_buffer.h"
 #include "gin/converter.h"
 #include "gin/public/isolate_holder.h"
+#include "gin/v8_initializer.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 #ifdef V8_USE_EXTERNAL_STARTUP_DATA
@@ -25,10 +26,10 @@ TEST(RunnerTest, Run) {
   std::string source = "this.result = 'PASS';\n";
 
 #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;