add SkCanvas constructor that explicitly creates no_config with dimensions

BUG=skia:
R=scroggo@google.com, halcanary@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/137833016

git-svn-id: http://skia.googlecode.com/svn/trunk@13272 2bbb7eff-a529-9590-31e7-b0007b416f81
7 files changed