MD Settings: Fix styles in several places.

chrome/browser/resources/settings/downloads_page/downloads_page.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="../controls/controlled_button.html">
 <link rel="import" href="../controls/settings_toggle_button.html">
 <link rel="import" href="../settings_shared_css.html">
 
 <dom-module id="settings-downloads-page">
   <template>
     <style include="settings-shared"></style>
     <div class="settings-box first two-line">
       <div class="start">
         <div>$i18n{downloadLocation}</div>
         <div class="secondary">
 <if expr="not chromeos">
           [[prefs.download.default_directory.value]]
 </if>
 <if expr="chromeos">
           [[getDownloadLocation_(prefs.download.default_directory.value)]]
 </if>
         </div>
       </div>
-      <div class="secondary-action">
-        <controlled-button class="secondary-button" id="changeDownloadsPath"
-            on-tap="selectDownloadLocation_"
-            pref="[[prefs.download.default_directory]]"
-            end-justified>
-          $i18n{changeDownloadLocation}

[dschuyler, 2017/02/01 23:06:23]

If this isn't used anymore, we can remove it from .cc file and the .grdp file
that reference the string.
('chrome/browser/ui/webui/settings/md_settings_localized_strings_provider.cc'
and 'chrome/app/settings_strings.grdp').

-        </controlled-button>
-      </div>
+      <controlled-button id="changeDownloadsPath" icon-class="icon-external"
+          on-tap="selectDownloadLocation_"
+          pref="[[prefs.download.default_directory]]"
+          end-justified>
+      </controlled-button>
     </div>
     <div class="settings-box block">
       <settings-toggle-button
           pref="{{prefs.download.prompt_for_download}}"
           label="$i18n{promptForDownload}">
       </settings-toggle-button>
 <if expr="chromeos">
       <settings-toggle-button
           pref="{{prefs.gdata.disabled}}"
           label="$i18n{disconnectGoogleDriveAccount}"
           hidden="[[!pageVisibility.googleDrive]]">
       </settings-toggle-button>
 </if>
     </div>
   </template>
   <script src="downloads_page.js"></script>
 </dom-module>