Add setting to isolate zoom changes by default.

chrome/browser/ui/zoom/chrome_zoom_level_prefs.cc

Index: chrome/browser/ui/zoom/chrome_zoom_level_prefs.cc
diff --git a/chrome/browser/ui/zoom/chrome_zoom_level_prefs.cc b/chrome/browser/ui/zoom/chrome_zoom_level_prefs.cc
index 65576ae8c34a1564339f73fcabfc979a81b829a7..88f79b11d7453265f65672a0f3fd162036c3811d 100644
--- a/chrome/browser/ui/zoom/chrome_zoom_level_prefs.cc
+++ b/chrome/browser/ui/zoom/chrome_zoom_level_prefs.cc
@@ -90,6 +90,34 @@ ChromeZoomLevelPrefs::RegisterDefaultZoomLevelCallback(
   return default_zoom_changed_callbacks_.Add(callback);
 }
 
+void ChromeZoomLevelPrefs::SetIsOriginScopePref(bool default_per_origin) {
+  if (default_per_origin == GetIsOriginScopePref())
+    return;
+
+  DictionaryPrefUpdate update(
+      pref_service_, prefs::kPartitionDefaultZoomOriginScope);
+  update->SetBoolean(partition_key_, default_per_origin);
+  default_scope_changed_callbacks_.Notify();
+}
+
+bool ChromeZoomLevelPrefs::GetIsOriginScopePref() const {
+  bool default_per_origin = true;

[wjmaclean, 2017/01/19 21:09:05]

Rename |default_per_origin| to something like |is_per_origin| to make it clear
we're not (always) using a default value here?

[Kevin McNee, 2017/02/03 22:51:34]

Done.

+
+  const base::DictionaryValue* default_zoom_origin_scope_dictionary =
+      pref_service_->GetDictionary(prefs::kPartitionDefaultZoomOriginScope);
+  // If no value has been previously set, |default_per_origin| will be untouched
+  // from the default value we've set above.
+  default_zoom_origin_scope_dictionary->GetBoolean(partition_key_,
+                                                   &default_per_origin);
+  return default_per_origin;
+}
+
+std::unique_ptr<ChromeZoomLevelPrefs::DefaultZoomScopeSubscription>
+ChromeZoomLevelPrefs::RegisterDefaultZoomScopeCallback(
+    const base::Closure& callback) {
+  return default_scope_changed_callbacks_.Add(callback);
+}
+
 void ChromeZoomLevelPrefs::OnZoomLevelChanged(
     const content::HostZoomMap::ZoomLevelChange& change) {
   // If there's a manager to aggregate ZoomLevelChanged events, pass this event