Don't create a gin::IsolateHolder in extensions unittests.

extensions/renderer/script_context_set_unittest.cc

Index: extensions/renderer/script_context_set_unittest.cc
diff --git a/extensions/renderer/script_context_set_unittest.cc b/extensions/renderer/script_context_set_unittest.cc
index 6b8849b54283f66baab8b6cc7eeb7a7713f0f32d..92eebd94af641613119e707a3080f43aba1549e4 100644
--- a/extensions/renderer/script_context_set_unittest.cc
+++ b/extensions/renderer/script_context_set_unittest.cc
@@ -8,7 +8,6 @@
 #include "extensions/renderer/script_context.h"
 #include "extensions/renderer/script_context_set.h"
 #include "gin/public/context_holder.h"
-#include "gin/public/isolate_holder.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/WebKit/public/web/WebFrame.h"
 #include "v8/include/v8.h"
@@ -21,7 +20,6 @@ TEST(ScriptContextSet, Lifecycle) {
   ScriptContextSet context_set;
 
   v8::Isolate* isolate = v8::Isolate::GetCurrent();
-  gin::IsolateHolder isolate_holder(isolate, NULL);
   v8::HandleScope handle_scope(isolate);
   gin::ContextHolder context_holder(isolate);
   context_holder.SetContext(v8::Context::New(isolate));