NTP: Sync shown sections across instances

This adds a pref observer for prefs::kNTPShownSections and calls a js function when these changes. This is so that instances of the NTP do not get out of sync with each other.

BUG=None
TEST=Open two or more NTPs. Hide and show different sections and the sections should be hidden and removed on all instances.

Review URL: http://codereview.chromium.org/2792004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@49487 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 20 insertions, 2 deletions

diff --git a/chrome/browser/resources/new_new_tab.html b/chrome/browser/resources/new_new_tab.html
index 3be5df2..b29a3fa 100644
--- a/chrome/browser/resources/new_new_tab.html
+++ b/chrome/browser/resources/new_new_tab.html
@@ -52,6 +52,7 @@ registerCallback('syncMessageChanged');
 registerCallback('tips');
 registerCallback('onHomePageSet');
 registerCallback('getAppsCallback');
+registerCallback('setShownSections');
 
 </script>
 <!-- template data placeholder -->
diff --git a/chrome/browser/resources/new_new_tab.js b/chrome/browser/resources/new_new_tab.js
index 16d47b7..8fc13a7 100644
--- a/chrome/browser/resources/new_new_tab.js
+++ b/chrome/browser/resources/new_new_tab.js
@@ -239,7 +239,9 @@ function getSectionElement(section) {
 function showSection(section) {
   if (!(section & shownSections)) {
     shownSections |= section;
-    getSectionElement(section).classList.remove('hidden');
+    var el = getSectionElement(section);
+    if (el)
+      el.classList.remove('hidden');
 
     switch (section) {
       case Section.THUMB:
@@ -267,7 +269,22 @@ function hideSection(section) {
         break;
     }
 
-    getSectionElement(section).classList.add('hidden');
+    var el = getSectionElement(section);
+    if (el)
+      el.classList.add('hidden');
+  }
+}
+
+/**
+ * Callback when the shown sections changes in another NTP.
+ * @param {number} newShownSections Bitmask of the shown sections.
+ */
+function setShownSections(newShownSections) {
+  for (var key in Section) {
+    if (newShownSections & Section[key])
+      showSection(Section[key]);
+    else
+      hideSection(Section[key]);
   }
 }