navBar: foreground-3 color for [disabled] selector

[rudzikdawid]

Bug, enhancement request, or proposal:

https://material.angularjs.org/HEAD/demo/navBar
Bug: color of disabled button._md-nav-button is the same as normal enabled button

Current master version:

Proposal:

PR: #11269

Example to reproduce the issue:

1. open https://material.angularjs.org/HEAD/demo/navBar
2. check "Disable Second Tab"

What is the expected behavior?

Color of disabled button should have value: '{{foreground-3}}'
inside navBar-theme.scss there is missing selector [disabled] ,
We should add something similar to tabs scss rules:

.md-tab {
    &[disabled] {
     color: '{{foreground-3}}';
    }

What is the current behavior?

color of disabled button._md-nav-button is the same as normal enabled button

What is the use-case or motivation for changing an existing behavior?

improove UX, adaptation to the material styleguide

Which versions of AngularJS, Material, OS, and browsers are affected?

• AngularJS: 1.6.10
• AngularJS Material: 1.1.9
• OS: Fedora
• Browsers: Chrome, Firefox

Is there anything else we should know? Stack Traces, Screenshots, etc.