use SkTaskGroup::batch

BUG=skia:

Review URL: https://codereview.chromium.org/688753002
diff --git a/src/core/SkMultiPictureDraw.cpp b/src/core/SkMultiPictureDraw.cpp
index defa295..fec272c 100644
--- a/src/core/SkMultiPictureDraw.cpp
+++ b/src/core/SkMultiPictureDraw.cpp
@@ -87,9 +87,7 @@
     // objects until after we're finished the tasks (which have pointers to the data).
 
     SkTaskGroup group;
-    for (int i = 0; i < fThreadSafeDrawData.count(); ++i) {
-        group.add(DrawData::Draw, &fThreadSafeDrawData[i]);
-    }
+    group.batch(DrawData::Draw, fThreadSafeDrawData.begin(), fThreadSafeDrawData.count());
     // we deliberately don't call wait() here, since the destructor will do that, this allows us
     // to continue processing gpu-data without having to wait on the cpu tasks.