Avoid Context::Enter and Context::Exit if we're reentering the active and last entered context

A bit of browsing around indicates that the new fast-path is taken most of the time:
   3496 Entering new
 152295 Reentering same

BUG=

Review-Url: https://codereview.chromium.org/2131483002
Cr-Commit-Position: refs/heads/master@{#37570}

src/api.cc

@@ -183,7 +183,17 @@ class CallDepthScope {
     DCHECK(!isolate_->external_caught_exception());
     isolate_->IncrementJsCallsFromApiCounter();
     isolate_->handle_scope_implementer()->IncrementCallDepth();
-    if (!context_.IsEmpty()) context_->Enter();
+    if (!context.IsEmpty()) {
+      i::Handle<i::Context> env = Utils::OpenHandle(*context);
+      i::HandleScopeImplementer* impl = isolate->handle_scope_implementer();
+      if (isolate->context() != nullptr &&
+          isolate->context()->native_context() == env->native_context() &&
+          impl->LastEnteredContextWas(env)) {
+        context_ = Local<Context>();
+      } else {
+        context_->Enter();
+      }
+    }
     if (do_callback_) isolate_->FireBeforeCallEnteredCallback();
   }
   ~CallDepthScope() {