MD Settings: Fix CrOS /help/details edge case for MainPageBehavior.

Without this, an assertion is triggered, since inSearchMode is updated
before the element is attached.

BUG=NONE
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2562153002
Cr-Commit-Position: refs/heads/master@{#438204}

chrome/browser/resources/settings/settings_page/main_page_behavior.js

@@ -66,6 +66,9 @@ var MainPageBehaviorImpl = {
    * @private
    */
   inSearchModeChanged_: function(inSearchMode) {
+    if (!this.isAttached)
+      return;
+
     if (!inSearchMode)
       this.tryTransitionToSection_(!settings.lastRouteChangeWasPopstate());
   },