RenderStyle should use copy-on-write inheritance for NinePieceImage.

RenderStyle should use copy-on-write inheritance for NinePieceImage.
<http://webkit.org/b/109366>

Reviewed by Antti Koivisto.

Refactor NinePieceImage to hold a copy-on-write DataRef like other RenderStyle substructures.
This allows us to avoids copying the NinePieceImageData when one RenderStyle inherits from another
but modifies something in the substructure holding the NinePieceImage (typically StyleSurroundData.)

Also made RenderStyle not copy-on-write its StyleSurroundData prematurely when doing a no-op write
to a border-image related value.

1.23 MB progression on Membuster3.

* rendering/style/NinePieceImage.cpp:
(WebCore::defaultData):
(WebCore::NinePieceImage::NinePieceImage):
(WebCore::NinePieceImageData::NinePieceImageData):
(WebCore::NinePieceImageData::operator==):
* rendering/style/NinePieceImage.h:
(WebCore::NinePieceImageData::create):
(WebCore::NinePieceImageData::copy):
(NinePieceImageData):
(NinePieceImage):
(WebCore::NinePieceImage::operator==):
(WebCore::NinePieceImage::operator!=):
(WebCore::NinePieceImage::hasImage):
(WebCore::NinePieceImage::image):
(WebCore::NinePieceImage::setImage):
(WebCore::NinePieceImage::imageSlices):
(WebCore::NinePieceImage::setImageSlices):
(WebCore::NinePieceImage::fill):
(WebCore::NinePieceImage::setFill):
(WebCore::NinePieceImage::borderSlices):
(WebCore::NinePieceImage::setBorderSlices):
(WebCore::NinePieceImage::outset):
(WebCore::NinePieceImage::setOutset):
(WebCore::NinePieceImage::horizontalRule):
(WebCore::NinePieceImage::setHorizontalRule):
(WebCore::NinePieceImage::verticalRule):
(WebCore::NinePieceImage::setVerticalRule):
(WebCore::NinePieceImage::copyImageSlicesFrom):
(WebCore::NinePieceImage::copyBorderSlicesFrom):
(WebCore::NinePieceImage::copyOutsetFrom):
(WebCore::NinePieceImage::copyRepeatFrom):
(WebCore::NinePieceImage::setMaskDefaults):
(WebCore::NinePieceImage::computeOutset):
* rendering/style/RenderStyle.cpp:
(WebCore::RenderStyle::setBorderImageSource):
(WebCore::RenderStyle::setBorderImageSlices):
(WebCore::RenderStyle::setBorderImageWidth):
(WebCore::RenderStyle::setBorderImageOutset):
* rendering/style/RenderStyle.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@142404 268f45cc-cd09-0410-ab3c-d52691b4dbfc