Remove GLContextStub::GetExtensions.

ui/gl/gl_context_stub.cc

 // 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.
 
 #include "ui/gl/gl_context_stub.h"
 
 namespace gfx {
 
 GLContextStub::GLContextStub() : GLContextReal(nullptr) {}
 
@@ skipping 16 matching lines @@
   return true;
 }
 
 void* GLContextStub::GetHandle() {
   return nullptr;
 }
 
 void GLContextStub::OnSetSwapInterval(int interval) {
 }
 
-std::string GLContextStub::GetExtensions() {
-  return std::string();
-}
-
 std::string GLContextStub::GetGLRenderer() {
   return std::string("CHROMIUM");
 }
 
 GLContextStub::~GLContextStub() {}
 
 }  // namespace gfx