[MD settings] margin around grid of avatar icons

This CL uses margins rather than max-width to determine when the avatar
icon grid layout should wrap.

BUG=646253
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2338003004
Cr-Commit-Position: refs/heads/master@{#419237}

chrome/browser/resources/settings/people_page/manage_profile.html

@@ -10,9 +10,7 @@
   <template>
     <style include="settings-shared">
       #selector {
-        -webkit-margin-start: 16px;
-        margin-top: 16px;
-        max-width: 624px;
+        margin: 16px 20px;
       }
     </style>
     <div class="settings-box first">