fix(icon): icons should have a minimum height.

* Icons should not shrink on smaller viewport. (At both axis, looks weird and is inconsistent)
* Also fixes an issue in the toolbar demo, where the icons shrink, when the viewport is to small.

Fixes #7599

Closes #7860

src/components/icon/icon.scss

@@ -7,6 +7,10 @@ md-icon {
  height: $icon-size;
  width: $icon-size;
 
+ // The icons should not shrink on smaller viewports.
+ min-height: $icon-size;
+ min-width: $icon-size;
+
  svg {
  pointer-events: none;
  display: block;