[MDCPageControl] Fixes MDCPageControl not properly revealing page indicators. (#6714)
[MDCPageControl] Fixes MDCPageControl not properly revealing page indicators. (#6714)

-[MDCPageControl revealIndicatorsReversed:] was incorrectly computing the indicator index when the reversed flag was YES. The index is updated to accommodate the reversal flag, but the collection is still being enumerated in normal order. This makes the isCurrentPageIndicator check apply to the wrong indicator.

|Before|After|
|---|---|
|![pc-rtl-bug](https://user-images.githubusercontent.com/1753199/53510109-3bd21680-3a8b-11e9-9405-e4001145b366.gif)|![pcfix](https://user-images.githubusercontent.com/4066863/53511623-d3853400-3a8e-11e9-9a52-d3345ae98254.gif)|
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios