Fix memory leak in SkGradientShader.

Make sure pointer to gradient shader cache is unreffed in SkGradientShaderBase::getGradientTableBitmap.
Rename methods returning a "pre-reffed" object to indicate obligations.

BUG=skia:1976
R=reed@google.com, scroggo@google.com

Author: dominikg@chromium.org

Review URL: https://codereview.chromium.org/240303003

git-svn-id: http://skia.googlecode.com/svn/trunk@14223 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed