Refactor IsolateHolder to be able to always create the isolate

mojo/apps/js/test/js_to_cpp_unittest.cc

Index: mojo/apps/js/test/js_to_cpp_unittest.cc
diff --git a/mojo/apps/js/test/js_to_cpp_unittest.cc b/mojo/apps/js/test/js_to_cpp_unittest.cc
index 206e266a7421280c27c9012cbe585025cfc3fdb3..5200bb71667d9c0cd3bf7f659c470b38ec8cb21a 100644
--- a/mojo/apps/js/test/js_to_cpp_unittest.cc
+++ b/mojo/apps/js/test/js_to_cpp_unittest.cc
@@ -8,6 +8,7 @@
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/utf_string_conversions.h"
+#include "gin/array_buffer.h"
 #include "gin/public/isolate_holder.h"
 #include "mojo/apps/js/mojo_runner_delegate.h"
 #include "mojo/apps/js/test/js_to_cpp.mojom.h"
@@ -398,7 +399,9 @@ class JsToCppTest : public testing::Test {
 
     cpp_side->set_js_side(js_side.get());
 
-    gin::IsolateHolder instance(gin::IsolateHolder::kStrictMode);
+    gin::IsolateHolder::Initialize(gin::IsolateHolder::kStrictMode,
+                                   gin::ArrayBufferAllocator::SharedInstance());
+    gin::IsolateHolder instance;
     apps::MojoRunnerDelegate delegate;
     gin::ShellRunner runner(&delegate, instance.isolate());
     delegate.Start(&runner, pipe.handle1.release().value(), test);