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/gpu/gl/GrGLContext.cpp b/src/gpu/gl/GrGLContext.cpp
index 296eaa5..54deb32 100644
--- a/src/gpu/gl/GrGLContext.cpp
+++ b/src/gpu/gl/GrGLContext.cpp
@@ -25,7 +25,7 @@
     this->reset();
     // We haven't validated the GrGLInterface yet, so check for GetString
     // function pointer
-    if (interface->fGetString) {
+    if (interface->fFunctions.fGetString) {
         const GrGLubyte* verUByte;
         GR_GL_CALL_RET(interface, verUByte, GetString(GR_GL_VERSION));
         const char* ver = reinterpret_cast<const char*>(verUByte);