In browser process, because of the use of v8::Locker in ProxyResolverV8,
creating HandleScope requires entering locker first, so in browser
process we should just use v8::Locker whenever we need to use V8 in a
callback called from non-V8 places (like dialog's callback, work pushed
to MessageLoop).

But also notice that the renderer process doesn't use v8::Locker, so we
have to be careful not to use it, otherwise blink would crash when
creating HandleScope.