Two visual fixes in the MD Clear Browsing Data dialog

1. Change the footer color to that of secondary text (i.e. --paper-grey-600).
2. Replace the 12px bottom margin with padding, so that this 12px high area
   at the bottom becomes a part of the scrollable region.

Screenshot: https://screenshot.googleplex.com/aodmwFZfdrq

BUG=595580
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2191283002
Cr-Commit-Position: refs/heads/master@{#408817}

chrome/browser/resources/settings/clear_browsing_data_dialog/clear_browsing_data_dialog.html

@@ -30,6 +30,7 @@
 
       #dialog .footer {
         background-color: #fff;
+        color: var(--paper-grey-600);
       }
 
       .row {
@@ -95,11 +96,15 @@
         display: inline;
       }
 
-      /* Cap the height on smaller screens to avoid unfavorable clipping. */
+      /* Cap the height on smaller screens to avoid unfavorable clipping.
+       * Replace the bottom margin with padding to avoid the gap between
+       * the scrollbar and the bottom separator. */
       @media all and (max-height: 714px) {
-        .body {
+        #dialog .body {
+          margin-bottom: 0;
           max-height: 270px;
           overflow-y: scroll;
+          padding-bottom: 12px;
         }
       }
     </style>