Change mobile viewport size to match Nexus 5X

[mattzeunert]

Lighthouse uses a resolution of 412x732 for the mobile setting, but we're trying to match the Nexus 5X viewport size which is only 412x660 (because of the phone chrome around it).

@patrickhulce said we might be able to change it. One upside would be that we could tell users to use Chrome's Nexus 5X emulation settings to test viewport-specific audit results, e.g. tap target sizes.