Revert of Make the Sk GL context class an abstract base class

tools/timer/GpuTimer.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 GpuTimer_DEFINED
 #define GpuTimer_DEFINED
 
-class SkGLContext;
+class SkGLContextHelper;
 
 class GpuTimer {
 public:
-    GpuTimer(const SkGLContext*);
+    GpuTimer(const SkGLContextHelper*);
     ~GpuTimer();
     void start();
     double end();
 private:
     unsigned fQuery;
     int fStarted;
-    const SkGLContext* fContext;
+    const SkGLContextHelper* fContext;
     bool fSupported;
 };
 
 #endif