build: a couple of minor improvements to the dev app (#21766)

Makes the following quality of life improvements to the dev app:
* Removes the horizontal scrolling on smaller screens which is caused by all
the buttons in the toolbar.
* Removes some extra spacing at the top of the page on IE.

Author: crisbeto

src/dev-app/dev-app/dev-app-layout.html

@@ -29,7 +29,7 @@
     be one level above the density class in the DOM. At the same time, we want the density to apply
     to the toolbar while always keeping it in LTR at the same time.
    -->
-  <main [attr.dir]="dir.value" [ngClass]="getDensityClass()">
+  <main [attr.dir]="dir.value" [ngClass]="getDensityClass()" class="demo-main">
     <!-- The toolbar should always be in the LTR direction -->
     <mat-toolbar color="primary" dir="ltr">
       <button mat-icon-button (click)="start.open('mouse')">

src/dev-app/dev-app/dev-app-layout.scss

@@ -41,6 +41,9 @@ body {
       justify-content: space-between;
       align-items: center;
       width: 100%;
+
+      // Avoids horizontal scrollbars on smaller screens.
+      overflow: hidden;
     }
   }
 
@@ -61,3 +64,8 @@ body {
   min-width: 100%;
   min-height: 100%;
 }
+
+.demo-main {
+  // Removes extra space on top of toolbar on IE.
+  display: block;
+}