fix(material-experimental/slider): incorrectly rendering extra background (#17207)
fix(material-experimental/slider): incorrectly rendering extra background (#17207)

Currently the slider always has the `mdc-slider--display-markers` class which causes an extra background to be rendered on sliders without markers. Since it's incorrect, these changes only add the class when markers are supposed to be rendered.
josephperrott/material2,angular/components,crisbeto/material2,Jefiozie/material2,josephperrott/material2,DevVersion/material2,andrewseguin/material2,Jefiozie/material2,mmalerba/material2,mmalerba/material2,andrewseguin/material2,andrewseguin/material2,mmalerba/material2,Jefiozie/material2,josephperrott/material2,angular/components,DevVersion/material2,crisbeto/material2,DevVersion/material2,josephperrott/material2,andrewseguin/material2,andrewseguin/material2,Jefiozie/material2,crisbeto/material2,josephperrott/material2,mmalerba/material2,DevVersion/material2,Jefiozie/material2,crisbeto/material2,DevVersion/material2,angular/components,angular/components,mmalerba/material2