Avoid printing draw target info to stderr while running unit tests

Change draw target dump function to return a SkString. Clients can do
whatever they want with the string.

BUG=skia:1837
R=caryclark@google.com, bsalomon@google.com

Author: kkinnunen@nvidia.com

Review URL: https://codereview.chromium.org/72353003

git-svn-id: http://skia.googlecode.com/svn/trunk/src@12340 2bbb7eff-a529-9590-31e7-b0007b416f81
5 files changed