Title mixin should use provide() for Vue 2.4.x

[aeharding]

I used the title mixin in my app with SSR and upgrading to 2.4.0 lead to the dreaded mergeData stack overflow exception.

Wrapping the static delaration in a provide() function fixes it.

https://github.com/vuejs/vue-hackernews-2.0/blob/master/src/util/title.js

vuejs/vue#6190

[aeharding]

Well, after some investigation, I can't get this to work when wrapping with provide(). I guess I don't understand mixins well enough. I also can't seem to duplicate the stack overflow with this project, but I can with mine (using the exact same mixin). Removing the title mixin from my project makes the error disappear as well.

Any advice is appreciated. For now, I guess I'm stuck on 2.3.x.