Add a helper function in TabObserver.java to handle WebContents Instant support disabled event.

Server side bug: b/17140979

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/532283003

Cr-Commit-Position: refs/heads/master@{#293182}

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

@@ -38,6 +38,9 @@ public class EmptyTabObserver implements TabObserver {
     @Override
     public void onContextMenuShown(Tab tab, ContextMenu menu) { }
 
+    @Override
+    public void onWebContentsInstantSupportDisabled() { }
+
     @Override
     public void onLoadStarted(Tab tabBase) { }
 

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

@@ -1025,7 +1025,8 @@ public class Tab implements NavigationClient {
      * disabled event.
      */
     @CalledByNative
-    protected void onWebContentsInstantSupportDisabled() {
+    private void onWebContentsInstantSupportDisabled() {
+      for (TabObserver observer : mObservers) observer.onWebContentsInstantSupportDisabled();
     }
 
     /**

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

@@ -78,6 +78,11 @@ public interface TabObserver {
      */
     void onContextMenuShown(Tab tab, ContextMenu menu);
 
+    /**
+     * Called when the WebContents Instant support is disabled.
+     */
+    void onWebContentsInstantSupportDisabled();
+
     // WebContentsDelegateAndroid methods ---------------------------------------------------------
 
     /**