Adding SK_API to SkGLContextHelper and SkNullGLContext for use in blink unit test component build

BUG=skia:
R=bsalomon@chromium.org

Author: junov@chromium.org

Review URL: https://codereview.chromium.org/141813002

git-svn-id: http://skia.googlecode.com/svn/trunk@13119 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed