Remove old TODO and commented-out DCHECK in Isolate::RunMicrotasks

Per discussion on the bug, the DCHECK'd case actually occurs in normal
operation, outside of V8's control.

BUG=chromium:388244
LOG=n

Review URL: https://codereview.chromium.org/1255043002

Cr-Commit-Position: refs/heads/master@{#29877}

src/isolate.cc

@@ -2653,13 +2653,6 @@ void Isolate::EnqueueMicrotask(Handle<Object> microtask) {
 
 
 void Isolate::RunMicrotasks() {
-  // %RunMicrotasks may be called in mjsunit tests, which violates
-  // this assertion, hence the check for --allow-natives-syntax.
-  // TODO(adamk): However, this also fails some layout tests.
-  //
-  // DCHECK(FLAG_allow_natives_syntax ||
-  //        handle_scope_implementer()->CallDepthIsZero());
-
   // Increase call depth to prevent recursive callbacks.
   v8::Isolate::SuppressMicrotaskExecutionScope suppress(
       reinterpret_cast<v8::Isolate*>(this));