fix blink::ToIsolate after r611992

https://chromium-review.googlesource.com/c/chromium/src/+/1252625
was supposed to do a DCHECK to ensure the semantics of ToIsolate()
remained unchanged, but the check was incorrect.  This fixes the semantics.

The next step is to remove ToIsolate(ExecutionContext).

Bug: 889542
Change-Id: Ic0a84fa74cafef47e43858f4dae4880b2f7757ec
Reviewed-on: https://chromium-review.googlesource.com/c/1355190Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612341}

third_party/blink/renderer/bindings/core/v8/v8_binding_for_core.cc

@@ -876,11 +876,15 @@ v8::Isolate* ToIsolate(const ExecutionContext* context) {
   if (!context)
     return nullptr;
 
+#if DCHECK_IS_ON()
   v8::Isolate* isolate;
   if (context && context->IsDocument())
     isolate = V8PerIsolateData::MainThreadIsolate();
-  isolate = v8::Isolate::GetCurrent();
+  else
+    isolate = v8::Isolate::GetCurrent();
   DCHECK(context->GetIsolate() == isolate);
+#endif
+
   return context->GetIsolate();
 }