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/DMPDFTask.h b/dm/DMPDFTask.h
index d107611..ae0dcce 100644
--- a/dm/DMPDFTask.h
+++ b/dm/DMPDFTask.h
@@ -24,7 +24,7 @@
 
     PDFTask(Reporter*,
             TaskRunner*,
-            SkPicture*,
+            const SkPicture*,
             SkString name,
             RasterizePdfProc);
 
@@ -37,7 +37,7 @@
 private:
     // One of these two will be set.
     SkAutoTDelete<skiagm::GM> fGM;
-    SkAutoTUnref<SkPicture> fPicture;
+    SkAutoTUnref<const SkPicture> fPicture;
 
     const SkString fName;
     RasterizePdfProc fRasterize;