fix(material/dialog): incorrect action height if box-sizing is configured globally (#20889)

It's common for apps to set `* { box-sizing: border-box; }` at a global level which ends up
breaking the styles of our dialog. These changes set an explicit `box-sizing` to ensure
that it still looks consistently. This is in line with what we've done in other similar cases.

Fixes #20887.

Author: crisbeto

src/material/dialog/dialog.scss

@@ -50,6 +50,10 @@ $mat-dialog-button-margin: 8px !default;
   min-height: 52px;
   align-items: center;
 
+  // Explicitly set a box-sizing since people commonly set `border-box`
+  // on all elements which will break the height of the dialog actions.
+  box-sizing: content-box;
+
   // Pull the actions down to avoid their padding stacking with the dialog's padding.
   margin-bottom: -$mat-dialog-padding;