get genmipmap function in the struct

BUG=
R=bsalomon@google.com, reed@google.com

Author: humper@google.com

Review URL: https://chromiumcodereview.appspot.com/20436002

git-svn-id: http://skia.googlecode.com/svn/trunk@10393 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/gl/GrGLCreateNullInterface.cpp b/src/gpu/gl/GrGLCreateNullInterface.cpp
index aa9b93c..0d539c9 100644
--- a/src/gpu/gl/GrGLCreateNullInterface.cpp
+++ b/src/gpu/gl/GrGLCreateNullInterface.cpp
@@ -109,6 +109,8 @@
     }
 }
 
+GrGLvoid GR_GL_FUNCTION_TYPE nullGLGenerateMipmap(GrGLenum target) {}
+
 GrGLvoid GR_GL_FUNCTION_TYPE nullGLBufferData(GrGLenum target,
                                               GrGLsizeiptr size,
                                               const GrGLvoid* data,
@@ -306,6 +308,7 @@
         interface->fFlush = noOpGLFlush;
         interface->fFrontFace = noOpGLFrontFace;
         interface->fGenBuffers = nullGLGenBuffers;
+        interface->fGenerateMipmap = nullGLGenerateMipmap;
         interface->fGenQueries = noOpGLGenIds;
         interface->fGenTextures = noOpGLGenIds;
         interface->fGenVertexArrays = noOpGLGenIds;