MD Settings: Fix subpage visibility and add appearance page tests

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

 <link rel="import" href="chrome://resources/html/util.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/web_ui_listener_behavior.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">
 <link rel="import" href="downloads_browser_proxy.html">
 
 <dom-module id="settings-downloads-page">
   <template>
@@ skipping 21 matching lines @@
           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>
+      <template is="dom-if" if="[[showPage_(pageVisibility.googleDrive)]]">

[dpapad, 2017/05/02 23:32:00]

Is it necessary to switch a div with |hidden| to dom-if (here and elsewhere in
this CL, I think I see such cases in appearance_page.js too).

[stevenjb, 2017/05/02 23:36:02]

Necessary? No. I thought it was preferred?

We couldn't use dom-if before because previously pageVisibility might not have
been defined.

[dpapad, 2017/05/02 23:45:52]

It is preferable when the dom-if wraps a large DOM subtree or expensive UI
components. Wrapping a fairly small subtree (imagine a single div with a dom-if)
can actually make things slower as opposed to using |hidden|.

Having said that, I have not audited all such changes in this CL, just
mentioning the overall principle on hidden vs dom-if I am aware of.

I am leaning towards preferring to keep the diff as minimal as possible for this
regression fix and consider changing hidden to dom-if as a separate optimization
CL if data supports it. 

Do you think it is too much extra work to revert those in this CL? If not, I am
fine with it, but it will make reviewing this CL a bit longer (since there is
more to review than just the fix).

[stevenjb, 2017/05/02 23:56:17]

I guess "large" is subjective.

I personally prefer consistency over pretty much everything, and some of the
sections (e.g. in the appearance page) are large.

That said, it's not that big a deal to switch back to 'hidden' everywhere if you
prefer. I don't feel strongly either way.

+        <settings-toggle-button
+            pref="{{prefs.gdata.disabled}}"
+            label="$i18n{disconnectGoogleDriveAccount}">
+        </settings-toggle-button>
+      </template>
 </if>
     </div>
     <template is="dom-if" if="[[autoOpenDownloads_]]" restamp>
       <div class="settings-box">
         <div class="start">
           $i18n{openFileTypesAutomatically}
         </div>
         <div class="separator"></div>
         <paper-button id="resetAutoOpenFileTypes" class="secondary-button"
             on-tap="onClearAutoOpenFileTypesTap_">
           $i18n{clear}
         </paper-button>
       </div>
     </template>
   </template>
   <script src="downloads_page.js"></script>
 </dom-module>