Add a max width to distilled pages

Review URL: https://codereview.chromium.org/481573002

Cr-Commit-Position: refs/heads/master@{#290141}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290141 0039d316-1c4b-4281-b951-d872f2087c98

components/dom_distiller/core/css/distilledpage.css

@@ -158,6 +158,11 @@ h6 {
 
 /* Main margins. */
 
+body {
+  max-width: 800px;
+  margin: 0px auto;
+}
+
 #mainContent {
   margin: 1.296rem 1.296rem auto;
 }