[Android O] Only delete legacy notification channels on upgrade

chrome/android/java/src/org/chromium/chrome/browser/notifications/ChannelsInitializer.java

Index: chrome/android/java/src/org/chromium/chrome/browser/notifications/ChannelsInitializer.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/notifications/ChannelsInitializer.java b/chrome/android/java/src/org/chromium/chrome/browser/notifications/ChannelsInitializer.java
index 526295fdd6e8206b691920b2edc36338ea76d3c0..299189c10f587f325c27e454bfc3e3f383e39c92 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/notifications/ChannelsInitializer.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/notifications/ChannelsInitializer.java
@@ -28,8 +28,12 @@ public class ChannelsInitializer {
         }
     }
 
-    void deleteAllChannels() {
-        for (String channelId : mNotificationManager.getNotificationChannelIds()) {
+    /**
+     * Cleans up any old channels that are no longer required from previous versions of the app.
+     * It's safe to call this multiple times since deleting an already-deleted channel is a no-op.
+     */
+    void deleteLegacyChannels() {
+        for (String channelId : mChannelDefinitions.getLegacyChannelIds()) {
             mNotificationManager.deleteNotificationChannel(channelId);
         }
     }