Removing getBackingTextureImage from ImageBufferSurface

Source/platform/graphics/ImageBufferSurface.h

Index: Source/platform/graphics/ImageBufferSurface.h
diff --git a/Source/platform/graphics/ImageBufferSurface.h b/Source/platform/graphics/ImageBufferSurface.h
index b6bbc276ceb00a6629efc6fbe70b0c7878e5ce66..c2be36514191f87e61d25b80b4ab62fa78649d5c 100644
--- a/Source/platform/graphics/ImageBufferSurface.h
+++ b/Source/platform/graphics/ImageBufferSurface.h
@@ -82,11 +82,8 @@ public:
     virtual void draw(GraphicsContext*, const FloatRect& destRect, const FloatRect& srcRect, SkXfermode::Mode);
     virtual void setHasExpensiveOp() { }
 
-    // These methods return an SkImage representing the backing texture. We support both variants
-    // to avoid the snapshot overhead (flushing deferred draws, COW semantics, fallback in
-    // RecordingImageBufferSurface) when not strictly required.
-    virtual PassRefPtr<SkImage> newImageSnapshot() const { return nullptr; }
-    virtual PassRefPtr<SkImage> getBackingTextureImage() const { return nullptr; }
+    // May return nullptr if the surface is GPU-backed and the GPU context was lost.
+    virtual PassRefPtr<SkImage> newImageSnapshot() const = 0;
 
     OpacityMode opacityMode() const { return m_opacityMode; }
     const IntSize& size() const { return m_size; }