Allow fallbackFactory to be null in RecordingImageBufferSurface.

third_party/WebKit/Source/platform/graphics/RecordingImageBufferSurface.h

Index: third_party/WebKit/Source/platform/graphics/RecordingImageBufferSurface.h
diff --git a/third_party/WebKit/Source/platform/graphics/RecordingImageBufferSurface.h b/third_party/WebKit/Source/platform/graphics/RecordingImageBufferSurface.h
index f7d37ae3a802ab2b8a6609e0bce7eb987dbc2bb5..7f87b6b5aa3333064a26810a413cbf193e990a2d 100644
--- a/third_party/WebKit/Source/platform/graphics/RecordingImageBufferSurface.h
+++ b/third_party/WebKit/Source/platform/graphics/RecordingImageBufferSurface.h
@@ -35,7 +35,11 @@ protected:
 class PLATFORM_EXPORT RecordingImageBufferSurface : public ImageBufferSurface {
     WTF_MAKE_NONCOPYABLE(RecordingImageBufferSurface); USING_FAST_MALLOC(RecordingImageBufferSurface);
 public:
-    RecordingImageBufferSurface(const IntSize&, PassOwnPtr<RecordingImageBufferFallbackSurfaceFactory> fallbackFactory, OpacityMode = NonOpaque);
+    // If the fallbackFactory is null the buffer surface should only be used
+    // for one frame and should not be used for any operations which need a
+    // raster surface, (i.e. writePixels).
+    // Only #getPicture should be used to access the resulting frame.
+    RecordingImageBufferSurface(const IntSize&, PassOwnPtr<RecordingImageBufferFallbackSurfaceFactory> fallbackFactory = nullptr, OpacityMode = NonOpaque);
     ~RecordingImageBufferSurface() override;
 
     // Implementation of ImageBufferSurface interfaces