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