[Cleanup] Avoid using deprecated v8::Function::New.

Moves to MaybeLocal version in api_event_handler.cc. This code should never through
when creating the function, so explicitly converting with ToLocalChecked().

BUG=v8:7290,v8:8238

Change-Id: I542384b457133b37d6279313bcabf874ad690591
Reviewed-on: https://chromium-review.googlesource.com/c/1349218Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610802}

extensions/renderer/bindings/api_event_handler.cc

@@ -281,8 +281,10 @@ void APIEventHandler::FireEventInContext(
     // We don't store this in a template because the Data (event name) is
     // different for each instance. Luckily, this is called during dispatching
     // an event, rather than e.g. at initialization time.
-    v8::Local<v8::Function> dispatch_event = v8::Function::New(
-        isolate, &DispatchEvent, gin::StringToSymbol(isolate, event_name));
+    v8::Local<v8::Function> dispatch_event =
+        v8::Function::New(context, &DispatchEvent,
+                          gin::StringToSymbol(isolate, event_name))
+            .ToLocalChecked();
 
     v8::Local<v8::Value> massager_args[] = {args_array, dispatch_event};
     JSRunner::Get(context)->RunJSFunction(