Implement new compositor and ContentViewCore reparenting for VR Shell.

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

Index: chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java b/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
index f97f467908e0ad2af404f8aed63c22a52cd2777b..2dc729448b16d0aa73b95e7ae81d4aba0a419fc8 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
@@ -1609,9 +1609,7 @@ public class Tab implements ViewGroup.OnHierarchyChangeListener,
         // TODO(yusufo): Share these calls with the construction related calls.
         // crbug.com/590281
 
-        // Update and propagate for the new WindowAndroid.
-        mWindowAndroid = activity.getWindowAndroid();
-        mContentViewCore.updateWindowAndroid(mWindowAndroid);
+        updateWindowAndroid(activity.getWindowAndroid());
 
         // Update for the controllers that need the Compositor from the new Activity.
         attachTabContentManager(activity.getTabContentManager());
@@ -1641,6 +1639,15 @@ public class Tab implements ViewGroup.OnHierarchyChangeListener,
     }
 
     /**
+     * Update and propagate the new WindowAndroid.
+     * @param windowAndroid The WindowAndroid to propagate.
+     */
+    public void updateWindowAndroid(WindowAndroid windowAndroid) {
+        mWindowAndroid = windowAndroid;
+        mContentViewCore.updateWindowAndroid(mWindowAndroid);
+    }
+
+    /**
      * @return Whether the tab is detached from its Activity and {@link WindowAndroid} for
      * reparenting. Certain functionalities will not work until it is attached to a new activity
      * with {@link Tab#attachAndFinishReparenting(