Install a hook to swap between SkPicture backends with a single define.

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

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/492023002
diff --git a/dm/DMUtil.cpp b/dm/DMUtil.cpp
index 92ce381..33af18e 100644
--- a/dm/DMUtil.cpp
+++ b/dm/DMUtil.cpp
@@ -26,7 +26,7 @@
     SkPictureRecorder recorder;
 
     SkCanvas* canvas = skr ? recorder.EXPERIMENTAL_beginRecording(w, h, factory)
-                           : recorder.             beginRecording(w, h, factory);
+                           : recorder.  DEPRECATED_beginRecording(w, h, factory);
     canvas->concat(gm->getInitialTransform());
     gm->draw(canvas);
     canvas->flush();