Move GrGLInterface function pointers into a nested struct

BUG=skia:2042
R=robertphillips@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/133073009

git-svn-id: http://skia.googlecode.com/svn/trunk@13130 2bbb7eff-a529-9590-31e7-b0007b416f81
18 files changed