Make WebWidget::compositeAndReadbackAsync() return void.

public/web/WebWidget.h

Index: public/web/WebWidget.h
diff --git a/public/web/WebWidget.h b/public/web/WebWidget.h
index 456e20e8508fef75b2dbfec3cdc8e433d45bedac..6681ed6b25880d19111d170b2806fcf67d6d0912 100644
--- a/public/web/WebWidget.h
+++ b/public/web/WebWidget.h
@@ -125,8 +125,9 @@ public:
     virtual void paintCompositedDeprecated(WebCanvas*, const WebRect&) { }
 
     // The caller is responsible for keeping the WebCompositeAndReadbackAsyncCallback
-    // object alive until it is called.
-    virtual bool compositeAndReadbackAsync(WebCompositeAndReadbackAsyncCallback*) { return false; }
+    // object alive until it is called. This should only be called when
+    // isAcceleratedCompositingActive() is true.
+    virtual void compositeAndReadbackAsync(WebCompositeAndReadbackAsyncCallback*) { }
 
     // Returns true if we've started tracking repaint rectangles.
     virtual bool isTrackingRepaints() const { return false; }