[cleanup][v8] Replace uses of deprecated API

R=mcilroy@chromium.org

Bug: v8:7295 v8:8015
Change-Id: I5de6e97a1e1ede6da0dc678579685bf5161abd35
Reviewed-on: https://chromium-review.googlesource.com/1237674Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595015}

gin/interceptor_unittest.cc

@@ -39,7 +39,10 @@ class MyInterceptor : public Wrappable<MyInterceptor>,
     if (property == "value") {
       return ConvertToV8(isolate, value_);
     } else if (property == "func") {
-      return GetFunctionTemplate(isolate, "func")->GetFunction();
+      v8::Local<v8::Context> context = isolate->GetCurrentContext();
+      return GetFunctionTemplate(isolate, "func")
+          ->GetFunction(context)
+          .ToLocalChecked();
     } else {
       return v8::Local<v8::Value>();
     }