Remove GLContextStub::GetExtensions.

ui/gl/gl_context_stub.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_GL_GL_CONTEXT_STUB_H_
 #define UI_GL_GL_CONTEXT_STUB_H_
 
 #include "base/macros.h"
 #include "ui/gl/gl_context.h"
 
 namespace gfx {
 
 // A GLContext that does nothing for unit tests.
 class GL_EXPORT GLContextStub : public GLContextReal {
  public:
   GLContextStub();
 
   // Implement GLContext.
   bool Initialize(GLSurface* compatible_surface,
                   GpuPreference gpu_preference) override;
   bool MakeCurrent(GLSurface* surface) override;
   void ReleaseCurrent(GLSurface* surface) override;
   bool IsCurrent(GLSurface* surface) override;
   void* GetHandle() override;
   void OnSetSwapInterval(int interval) override;
-  std::string GetExtensions() override;
   std::string GetGLRenderer() override;
 
  protected:
   ~GLContextStub() override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(GLContextStub);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GL_GL_CONTEXT_STUB_H_