Deprecate SkPicture::clone().

Chrome will need -DSK_SUPPORT_LEGACY_PICTURE_CLONE.

This removes the modes from our tools that use clone().  No
bots run these.  DM used clone() in a way that we can just
share the picture now.

I plan to bring back the ability to test multithreaded
picture rendering soon.

BUG=skia:2378
R=robertphillips@google.com, mtklein@google.com, bsalomon@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/338633011
diff --git a/dm/DM.cpp b/dm/DM.cpp
index 2981013..64a4bc5 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -186,8 +186,8 @@
             exit(1);
         }
 
-        tasks->add(SkNEW_ARGS(DM::SKPTask, (reporter, tasks, pic->clone(), filename)));
-        tasks->add(SkNEW_ARGS(DM::PDFTask, (reporter, tasks, pic->clone(), filename,
+        tasks->add(SkNEW_ARGS(DM::SKPTask, (reporter, tasks, pic, filename)));
+        tasks->add(SkNEW_ARGS(DM::PDFTask, (reporter, tasks, pic, filename,
                                             RASTERIZE_PDF_PROC)));
     }
 }