Update from https://crrev.com/316786

mojo/cc/context_provider_mojo.h

 // Copyright 2014 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 MOJO_CC_CONTEXT_PROVIDER_MOJO_H_
 #define MOJO_CC_CONTEXT_PROVIDER_MOJO_H_
 
 #include "base/macros.h"
+#include "base/synchronization/lock.h"
 #include "cc/output/context_provider.h"
 #include "mojo/public/c/gles2/gles2.h"
 #include "mojo/public/cpp/system/core.h"
 
 namespace mojo {
 
 class ContextProviderMojo : public cc::ContextProvider {
  public:
   explicit ContextProviderMojo(ScopedMessagePipeHandle command_buffer_handle);
 
   // cc::ContextProvider implementation.
   bool BindToCurrentThread() override;
   gpu::gles2::GLES2Interface* ContextGL() override;
   gpu::ContextSupport* ContextSupport() override;
   class GrContext* GrContext() override;
   Capabilities ContextCapabilities() override;
   bool IsContextLost() override;
   void VerifyContexts() override {}
   void DeleteCachedResources() override {}
   bool DestroyedOnMainThread() override;
   void SetLostContextCallback(
       const LostContextCallback& lost_context_callback) override {}
   void SetMemoryPolicyChangedCallback(
       const MemoryPolicyChangedCallback& memory_policy_changed_callback)
       override {}
+  void SetupLock() override;
+  base::Lock* GetLock() override;
 
  protected:
   friend class base::RefCountedThreadSafe<ContextProviderMojo>;
   ~ContextProviderMojo() override;
 
  private:
   static void ContextLostThunk(void* closure) {
     static_cast<ContextProviderMojo*>(closure)->ContextLost();
   }
   void ContextLost();
 
   cc::ContextProvider::Capabilities capabilities_;
   ScopedMessagePipeHandle command_buffer_handle_;
   MojoGLES2Context context_;
   bool context_lost_;
 
+  base::Lock context_lock_;
+
   DISALLOW_COPY_AND_ASSIGN(ContextProviderMojo);
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_CC_CONTEXT_PROVIDER_MOJO_H_