Revert of Allow canvases to be GPU-accelerated in Workers

third_party/WebKit/Source/platform/graphics/gpu/SharedGpuContext.h

-// Copyright 2016 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 "wtf/ThreadSpecific.h"
-
-#include <memory>
-
-namespace gpu { namespace gles2 { class GLES2Interface; } }
-class GrContext;
-
-namespace blink {
-
-class WaitableEvent;
-class WebGraphicsContext3DProvider;
-
-// SharedGpuContext provides access to a thread-specific GPU context
-// that is shared by many callsites throughout the thread.
-// When on the main thread, provides access to the same context as
-// Platform::createSharedOffscreenGraphicsContext3DProvider
-class SharedGpuContext {
-public:
-    // The contextId is incremented each time a new underlying context
-    // is created. For example, when the context is lost, then restored.
-    // User code can rely on this Id to determine whether long-lived
-    // gpu resources are still alive in the current context.
-    static unsigned contextId();
-    static gpu::gles2::GLES2Interface* gl();
-    static GrContext* gr();
-    static bool isValid();
-    static bool restore();
-
-    enum {
-        kNoSharedContext = 0,
-    };
-
-private:
-    static SharedGpuContext* getInstanceForCurrentThread();
-
-    SharedGpuContext();
-    void createContextProviderOnMainThread(WaitableEvent*);
-    void createContextProvider();
-
-    std::unique_ptr<WebGraphicsContext3DProvider> m_contextProvider;
-    unsigned m_contextId;
-    friend class WTF::ThreadSpecific<SharedGpuContext>;
-};
-
-} // blink