canvas: make a temporary buffer when a context doesn't exist.

Source/core/css/CSSCanvasValue.cpp

Index: Source/core/css/CSSCanvasValue.cpp
diff --git a/Source/core/css/CSSCanvasValue.cpp b/Source/core/css/CSSCanvasValue.cpp
index 1c089e642f2702dc5b02cb83ffeeb865493511f6..5d776799a78e3039108152211fbca6d6d03d75bf 100644
--- a/Source/core/css/CSSCanvasValue.cpp
+++ b/Source/core/css/CSSCanvasValue.cpp
@@ -89,7 +89,7 @@ PassRefPtr<Image> CSSCanvasValue::image(RenderObject* renderer, const IntSize& /
 {
     ASSERT(clients().contains(renderer));
     HTMLCanvasElement* elt = element(&renderer->document());
-    if (!elt || !elt->buffer())
+    if (!elt)

[dshwang, 2014/11/27 15:12:11]

I remove all buffer() usages from other modules
Now HTMLCanvasElement::buffer() is called by only canvas code. It makes
refactorying code about imagebuffer easier.

         return nullptr;
     return elt->copiedImage(FrontBuffer);
 }