add dummy onDrawPicture to allow migration for chrome overrides

after this lands in chrome, land these two CLs
https://codereview.chromium.org/454993002/
https://codereview.chromium.org/454143003

then land this skia CL
https://codereview.chromium.org/448793004/

then remove the onDrawPicture overrides from chrome

R=fmalita@google.com, robertphillips@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/457753002

include/core/SkCanvas.h

diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index f74ebb7..eef9d90 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -1224,6 +1224,8 @@
     virtual void onDiscard();
 
     virtual void onDrawPicture(const SkPicture* picture);
+    // temporary, until we can migrate existing overrides in chrome
+    virtual void onDrawPicture(const SkPicture*, const SkMatrix*, const SkPaint*) {}
 
     // Returns the canvas to be used by DrawIter. Default implementation
     // returns this. Subclasses that encapsulate an indirect canvas may