Now that CreateFromStream is used, check for NULL rather than
looking at width and height (prevents a crash on an invalid SKP).
Unref the picture when done with it.
Remove the member variable for picture, since it is only accessed
inside the one function.
R=borenet@google.com
Review URL: https://codereview.chromium.org/18055014
git-svn-id: http://skia.googlecode.com/svn/trunk@9831 2bbb7eff-a529-9590-31e7-b0007b416f81
