ambient: Put ambient mode subpage behind feature

This patch wraps the ambient mode subpage path in Chrome OS settings behind the
ambient mode feature.

Bug: 1068816
Test: manual
Change-Id: I7e1cdf2dabdc7da15d818304ee6d18c231b99a59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2142140Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757997}

chrome/browser/resources/settings/chromeos/personalization_page/personalization_page.html

@@ -55,13 +55,16 @@
           <settings-change-picture></settings-change-picture>
         </settings-subpage>
       </template>
-      <template is="dom-if" route-path="/ambientMode">
-        <settings-subpage
-            associated-control="[[$$('#ambientModeRow')]]"
-            page-title="$i18n{ambientModeTitle}">
-          <settings-ambient-mode-page prefs="{{prefs}}">
-          </settings-ambient-mode-page>
-        </settings-subpage>
+
+      <template is="dom-if" if="[[isAmbientModeEnabled_]]">
+        <template is="dom-if" route-path="/ambientMode">
+          <settings-subpage
+              associated-control="[[$$('#ambientModeRow')]]"
+              page-title="$i18n{ambientModeTitle}">
+            <settings-ambient-mode-page prefs="{{prefs}}">
+            </settings-ambient-mode-page>
+          </settings-subpage>
+        </template>
       </template>
     </settings-animated-pages>
   </template>