Bug 221550

Before r269614, the pattern with SVG image was not rendered at all. After this revision the pattern is drawn as a black rectangle. The reason is with r269614 Pattern::createPlatformPattern() returns a null CGPatternRef if the NativeImage of the SVG is null. Before this change we were returning a non-null CGPatternRef but with an invalid image. The test imported/blink/svg/canvas/canvas-draw-pattern-size.html did not catch this case before this change or after it, because the expected result page draws a pattern but with a different size. The fix of this bug is implement the method SVGImage::nativeImage(). Also we need to change the expected result imported/blink/svg/canvas/canvas-draw-pattern-size-expected.html to not use the pattern in its drawing.