Center distiller results on desktop

This change centers the distiller results page. This will only
affect the desktop version and is purely for developer sanity
as desktop distiller is not officially supported.

BUG=

Review URL: https://codereview.chromium.org/1109853002

Cr-Commit-Position: refs/heads/master@{#327090}

components/dom_distiller/core/css/distilledpage.css

@@ -167,7 +167,7 @@ h6 {
 
 body {
   max-width: 800px;
-  margin: 0px;
+  margin: 0px auto 0px auto;
 }
 
 #mainContent {