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
diff --git a/src/views/win/SkOSWindow_win.cpp b/src/views/win/SkOSWindow_win.cpp
index e91d73a..027bc66 100644
--- a/src/views/win/SkOSWindow_win.cpp
+++ b/src/views/win/SkOSWindow_win.cpp
@@ -27,7 +27,7 @@
 
 #define ANGLE_GL_CALL(IFACE, X)                                 \
     do {                                                        \
-        (IFACE)->f##X;                                          \
+        (IFACE)->fFunctions.f##X;                               \
     } while (false)
 
 #endif