Support multiple null GL contexts on a thread.

include/gpu/gl/GrGLInterface.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLInterface_DEFINED
 #define GrGLInterface_DEFINED
 
@@ skipping 38 matching lines @@
 #endif
 
 #if SK_ANGLE
 /**
  * Creates a GrGLInterface for an ANGLE context.
  */
 const GrGLInterface* GrGLCreateANGLEInterface();
 #endif
 
 /**
- * Creates a null GrGLInterface that doesn't draw anything. Used for measuring
- * CPU overhead.
- */
-const SK_API GrGLInterface* GrGLCreateNullInterface();
-
-/**
  * Creates a debugging GrGLInterface that doesn't draw anything. Used for
  * finding memory leaks and invalid memory accesses.
  */
 const GrGLInterface* GrGLCreateDebugInterface();
 
 #if GR_GL_PER_GL_FUNC_CALLBACK
 typedef void (*GrGLInterfaceCallbackProc)(const GrGLInterface*);
 typedef intptr_t GrGLInterfaceCallbackData;
 #endif
 
@@ skipping 279 matching lines @@
 #if GR_GL_PER_GL_FUNC_CALLBACK
     GrGLInterfaceCallbackProc fCallback;
     GrGLInterfaceCallbackData fCallbackData;
 #endif
 
     // This exists for internal testing.
     virtual void abandon() const {}
 };
 
 #endif