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.
It's not necessary to create HandleScope in v8 native binding functions,
because it's guarrenteed that the there is a upper HandleScope when the
native function is called. So unless we are urgent to free the handles,
not using HandleScope in native binding functions is a cleaner and safer
choice.
