fix(progressCircular): fixes animation bug when used inside md-dialog

Closes #5039

Author: Robert Messerle

src/components/progressCircular/progress-circular.scss

@@ -17,6 +17,7 @@ md-progress-circular {
   margin-bottom: 0 !important;
 
   transform: scale(0.5);
+  z-index: $z-index-progress-circular; // Used to fix a Chrome bug with spinner animation
 
   .md-spinner-wrapper {
     display:block;

src/core/style/variables.scss

@@ -83,6 +83,7 @@ $z-index-scroll-mask: 65 !default;
 $z-index-sidenav: 60 !default;
 $z-index-backdrop: 50 !default;
 $z-index-fab: 20 !default;
+$z-index-progress-circular: 2 !default; // Used to fix animation bug in Chrome
 
 // Easing Curves
 //--------------------------------------------