Fix crash in ChromeZoomLevelPrefs::ExtractPerHostZoomLevels().

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 08879def07c1dc960e1f442cf673f1844a61ca1a..d9092df2e98f5302f1c11df64fedfadf481cd39b 100644
--- a/chrome/browser/ui/zoom/chrome_zoom_level_prefs.cc
+++ b/chrome/browser/ui/zoom/chrome_zoom_level_prefs.cc
@@ -78,7 +78,8 @@ void ChromeZoomLevelPrefs::InitPrefsAndCopyToHostZoomMap(
     // Since we're calling this before setting up zoom_subscription_ below we
     // don't need to worry that host_zoom_dictionary is indirectly affected
     // by calls to HostZoomMap::SetZoomLevelForHost().
-    ExtractPerHostZoomLevels(host_zoom_dictionary);
+    ExtractPerHostZoomLevels(host_zoom_dictionary,
+                             prefs::kPartitionPerHostZoomLevels);
   }
   zoom_subscription_ = host_zoom_map_->AddZoomLevelChangedCallback(base::Bind(
       &ChromeZoomLevelPrefs::OnZoomLevelChanged, base::Unretained(this)));
@@ -144,8 +145,11 @@ void ChromeZoomLevelPrefs::OnZoomLevelChanged(
     host_zoom_dictionary->SetDoubleWithoutPathExpansion(change.host, level);
 }
 
+// TODO(wjmaclean): Remove the dictionary_path once the migration code is
+// removed. crbug.com/420643
 void ChromeZoomLevelPrefs::ExtractPerHostZoomLevels(
-    const base::DictionaryValue* host_zoom_dictionary) {
+    const base::DictionaryValue* host_zoom_dictionary,
+    const char* dictionary_path) {
   std::vector<std::string> keys_to_remove;
   scoped_ptr<base::DictionaryValue> host_zoom_dictionary_copy(
       host_zoom_dictionary->DeepCopyWithoutEmptyChildren());
@@ -174,11 +178,18 @@ void ChromeZoomLevelPrefs::ExtractPerHostZoomLevels(
     host_zoom_map_->SetZoomLevelForHost(host, zoom_level);
   }
 
+  // We don't bother sanitizing non-partition dictionaries as they will be
+  // discarded in the migration process. Note: since the structure of partition
+  // per-host zoom level dictionaries is different from the legacy profile
+  // per-host zoom level dictionaries, the following code will fail if run
+  // on the legacy dictionaries.
+  if (strcmp(dictionary_path, prefs::kPartitionPerHostZoomLevels) != 0)
+    return;
+
   // Sanitize prefs to remove entries that match the default zoom level and/or
   // have an empty host.
   {
-    DictionaryPrefUpdate update(pref_service_,
-                                prefs::kPartitionPerHostZoomLevels);
+    DictionaryPrefUpdate update(pref_service_, dictionary_path);
     base::DictionaryValue* host_zoom_dictionaries = update.Get();
     base::DictionaryValue* host_zoom_dictionary = nullptr;
     host_zoom_dictionaries->GetDictionary(partition_key_,