Make CanvasRenderingContext2D use SkCanvas directly

Source/platform/graphics/ImageBuffer.cpp

Index: Source/platform/graphics/ImageBuffer.cpp
diff --git a/Source/platform/graphics/ImageBuffer.cpp b/Source/platform/graphics/ImageBuffer.cpp
index 5324dbade0027a5c4c0d2bb8d33ace568d65a1ce..b2646e0d0b50735fd3aa3e50aafb88557960bc5c 100644
--- a/Source/platform/graphics/ImageBuffer.cpp
+++ b/Source/platform/graphics/ImageBuffer.cpp
@@ -95,11 +95,18 @@ ImageBuffer::~ImageBuffer()
 GraphicsContext* ImageBuffer::context() const
 {
     if (!isSurfaceValid())
-        return 0;
+        return nullptr;
     ASSERT(m_context.get());
     return m_context.get();
 }
 
+SkCanvas* ImageBuffer::canvas() const
+{
+    if (!isSurfaceValid())
+        return nullptr;
+    return m_surface->canvas();
+}
+
 const SkBitmap& ImageBuffer::bitmap() const
 {
     return m_surface->bitmap();