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

content/public/test/DEPS

Index: content/public/test/DEPS
diff --git a/content/public/test/DEPS b/content/public/test/DEPS
index 43a7d3ceb2b33535e6e1d14f0e5a4854a5e0abe3..e897ad3c397d81de21882c64ecfede2d8c1ce00f 100644
--- a/content/public/test/DEPS
+++ b/content/public/test/DEPS
@@ -9,7 +9,7 @@ specific_include_rules = {
   ".*\.cc": [
     # Testing utilities can access anything in content/
     "+content",
-    "+gin/public/isolate_holder.h",
+    "+gin/v8_initializer.h",
     "+media/base",
     "+third_party/iaccessible2",
     "+ui/base/resource/resource_bundle.h",