Make render_pdfs not crash webpages_playback script

The render_pdfs nowadays seems to input the skps from
the dir passed as '-r' parameter.

The app will not output anything, but it will not crash the
webpages_playback script either. Affects only runs that have
--skia_tools=<dir> argument.

Review URL: https://codereview.chromium.org/776873004

tools/skp/webpages_playback.py

@@ -295,7 +295,7 @@ class SkPicturePlayback(object):
       ]
       render_pdfs_cmd = [
           os.path.join(self._skia_tools, 'render_pdfs'),
-          self._local_skp_dir
+          '-r', self._local_skp_dir
       ]
 
       for tools_cmd in (render_pictures_cmd, render_pdfs_cmd):