Rollup merge of #56959 - JohnHeitmann:mobile-z-fix, r=GuillaumeGomez
Rollup merge of #56959 - JohnHeitmann:mobile-z-fix, r=GuillaumeGomez

Fix mobile menu rendering collision with tooltip.

Bring the mobile-mode menu in front of the ⓘ icon.

Here's what the bug looks like:

![screen shot 2018-12-18 at 1 53 46 pm](https://user-images.githubusercontent.com/4282480/50185501-cbf62180-02cc-11e9-927e-3c6469901323.png)

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust