WebUI: restrict cr-dialogs max-height to the viewport.

ui/webui/resources/cr_elements/cr_dialog/cr_dialog.html

 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/html/assert.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/default-theme.html">
 
 <dom-module id="cr-dialog">
   <template>
     <style>
       :host {
         --scroll-border: 1px solid var(--paper-grey-300);
         border: 0;
         border-radius: 2px;
         bottom: 0;
         box-shadow: 0px 0px 16px rgba(0, 0, 0, 0.12),
                     0px 16px 16px rgba(0, 0, 0, 0.24);
         color: inherit;
         padding: 0;
         top: 0;
         width: 512px;
       }
 
+      :host([open]) {
+        /* Keep max-height within viewport, and flex content accordingly. */
+        display: flex;
+        flex-direction: column;
+        max-height: 100vh;
+      }
+
+      /* When needing to flex, force .body-container alone to shrink. */
+      .top-container,
+      :host ::content .button-container,
+      :host ::content .footer {
+        flex-shrink: 0;
+      }
+
       :host::backdrop {
         background-color: rgba(0, 0, 0, 0.6);
         bottom: 0;
         left: 0;
         position: fixed;
         right: 0;
         top: 0;
       }
 
       :host([show-scroll-borders]) .body-container {
@@ skipping 39 matching lines @@
         padding: 16px 20px;
       }
 
       :host ::content .border-top-divider {
         border-top: 1px solid var(--divider-color);
       }
 
       .body-container {
         display: flex;
         flex-direction: column;
+        min-height: 60px; /* Minimum reasonably usable height. */
         overflow: auto;
         @apply(--cr-dialog-body-container);
       }
 
       .top-container {
         align-items: flex-start;
         display: flex;
         min-height: 47px;
       }
 
@@ skipping 29 matching lines @@
     <div class="body-container">
       <span id="bodyTopMarker"></span>
       <content select=".body"></content>
       <span id="bodyBottomMarker"></span>
     </div>
     <content select=".button-container"></content>
     <content select=".footer"></content>
   </template>
   <script src="cr_dialog.js"></script>
 </dom-module>