[d8] Update realm_current_ and realm_switch_ in Realm.eval

This makes e.g., load(file) work within Realm.eval(realm, "load(file)") to load files into that realm.

Bug:

Change-Id: I85738f0dfab621f2a8c9e2703f4ce4b39dd882bf
Reviewed-on: https://chromium-review.googlesource.com/484379Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44887}

src/d8.cc

@@ -1076,12 +1076,16 @@ void Shell::RealmEval(const v8::FunctionCallbackInfo<v8::Value>& args) {
   }
   Local<Context> realm = Local<Context>::New(isolate, data->realms_[index]);
   realm->Enter();
+  int previous_index = data->realm_current_;
+  data->realm_current_ = data->realm_switch_ = index;
   Local<Value> result;
   if (!script->BindToCurrentContext()->Run(realm).ToLocal(&result)) {
     realm->Exit();
+    data->realm_current_ = data->realm_switch_ = previous_index;
     return;
   }
   realm->Exit();
+  data->realm_current_ = data->realm_switch_ = previous_index;
   args.GetReturnValue().Set(result);
 }