allow createScalerContext to return null, and then have the GDI backend trigger that if we fail to create the necessary GDI resources

BUG=
R=bungeman@google.com

Review URL: https://codereview.chromium.org/18484005

git-svn-id: http://skia.googlecode.com/svn/trunk@9957 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed