Add a flag to flush the canvases during SkMultiPictureDraw::draw().

src/core/SkMultiPictureDraw.cpp

Index: src/core/SkMultiPictureDraw.cpp
diff --git a/src/core/SkMultiPictureDraw.cpp b/src/core/SkMultiPictureDraw.cpp
index c85c56735ccd64cfdfe20cfa7de3df7709c96ca4..7cdab7300c3253d4cd2e7b489fa0cca74c22eab5 100644
--- a/src/core/SkMultiPictureDraw.cpp
+++ b/src/core/SkMultiPictureDraw.cpp
@@ -86,7 +86,7 @@ public:
 
 //#define FORCE_SINGLE_THREAD_DRAWING_FOR_TESTING
 
-void SkMultiPictureDraw::draw() {
+void SkMultiPictureDraw::draw(bool flush) {
     AutoMPDReset mpdreset(this);
 
 #ifdef FORCE_SINGLE_THREAD_DRAWING_FOR_TESTING
@@ -192,6 +192,9 @@ void SkMultiPictureDraw::draw() {
         {
             canvas->drawPicture(picture, &data.fMatrix, data.fPaint);
         }
+        if (flush) {
+            canvas->flush();
+        }
     }
 
 #if !defined(SK_IGNORE_GPU_LAYER_HOISTING) && SK_SUPPORT_GPU