[Dialog] Add support for a Dialog without a DialogTitle (mui#16980)

* Fix dialog content padding without title

* even closer to spec

Author: megos

packages/material-ui/src/DialogContent/DialogContent.js

@@ -7,12 +7,17 @@ export const styles = theme => ({
   /* Styles applied to the root element. */
   root: {
     flex: '1 1 auto',
-    padding: '8px 24px',
     WebkitOverflowScrolling: 'touch', // Add iOS momentum scrolling.
     overflowY: 'auto',
+    padding: '8px 24px',
+    '&:first-child': {
+      // dialog without title
+      paddingTop: 20,
+    },
   },
   /* Styles applied to the root element if `dividers={true}`. */
   dividers: {
+    padding: '16px 24px',
     borderTop: `1px solid ${theme.palette.divider}`,
     borderBottom: `1px solid ${theme.palette.divider}`,
   },

packages/material-ui/src/DialogContentText/DialogContentText.js

@@ -6,7 +6,7 @@ import Typography from '../Typography';
 export const styles = {
   /* Styles applied to the root element. */
   root: {
-    marginBottom: 16,
+    marginBottom: 12,
   },
 };