Introduce ViewRoot to forward input/view events to native

ui/android/java/src/org/chromium/ui/base/WindowAndroid.java

Index: ui/android/java/src/org/chromium/ui/base/WindowAndroid.java
diff --git a/ui/android/java/src/org/chromium/ui/base/WindowAndroid.java b/ui/android/java/src/org/chromium/ui/base/WindowAndroid.java
index fc60dd20831b3b554d11e1b868ea753fab43c084..94d70acdad327c4e551b111053a5f38fbb04b92d 100644
--- a/ui/android/java/src/org/chromium/ui/base/WindowAndroid.java
+++ b/ui/android/java/src/org/chromium/ui/base/WindowAndroid.java
@@ -109,6 +109,8 @@ public class WindowAndroid {
 
     private AndroidPermissionDelegate mPermissionDelegate;
 
+    private EventHandler mEventHandler;
+
     /**
      * An interface to notify listeners of changes in the soft keyboard's visibility.
      */
@@ -687,6 +689,16 @@ public class WindowAndroid {
         }
     }
 
+    /**
+     * @return {@link EventHandler} instance used to forward input/view events down to native.
+     */
+    public EventHandler getEventHandler() {
+        if (mEventHandler == null) {
+            mEventHandler = new EventHandler(mNativeWindowAndroid);

[boliu, 2016/12/06 23:15:46]

creating EventHandler is unnecessarily duplicated.. is it possible to have
ViewAndroid create the java object and return a JavaRef to it?

[Jinsuk Kim, 2016/12/07 12:36:28]

Done. I found that it more convenient to create it in native WindowAndroid.
Chrome comes through WA java to the native, and WebView reaches it via
AwContents -> ViewAndroid -> WA. 

Now that VA keeps the weak reference to Java ref, it replaced the prev boolean
|has_event_handler_| for checking the presence of handler for each VA.  

+        }
+        return mEventHandler;
+    }
+
     private native long nativeInit(int displayId);
     private native void nativeOnVSync(long nativeWindowAndroid,
                                       long vsyncTimeMicros,