Add support for ContentViewCore helpers on Tab

chrome/android/java/src/org/chromium/chrome/browser/EmptyTabObserver.java

Index: chrome/android/java/src/org/chromium/chrome/browser/EmptyTabObserver.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/EmptyTabObserver.java b/chrome/android/java/src/org/chromium/chrome/browser/EmptyTabObserver.java
index 80f1eb219dc54146286725456158c47d5c3ecb4d..dce9c637f4206a5ed8778015599767f6606fab85 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/EmptyTabObserver.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/EmptyTabObserver.java
@@ -6,6 +6,7 @@ package org.chromium.chrome.browser;
 
 import android.view.ContextMenu;
 
+import org.chromium.content.browser.ContentViewCore;
 import org.chromium.content_public.browser.WebContents;
 
 /**
@@ -29,6 +30,12 @@ public class EmptyTabObserver implements TabObserver {
     public void onContentChanged(Tab tab) { }
 
     @Override
+    public void onOverlayContentViewCoreAdded(Tab tab, ContentViewCore content) { }
+
+    @Override
+    public void onOverlayContentViewCoreRemoved(Tab tab, ContentViewCore content) { }
+
+    @Override
     public void onLoadUrl(Tab tab, String url, int loadType) { }
 
     @Override