StylePendingImage needs to correctly manage the CSSValue pointer lifetime

https://bugs.webkit.org/show_bug.cgi?id=125468

Reviewed by Andreas Kling.

Source/WebCore:

Test: fast/css/pending-image-crash.xhtml

Disconnect the reference counted StylePendingImage from the CSSValue that owns
it when it's not needed any more, otherwise we could end up using a pointer
that might no longer be valid.

* css/CSSCursorImageValue.cpp:
(WebCore::CSSCursorImageValue::detachPendingImage): Added. Calls detachFromCSSValue
on the current image if it is a StylePendingImage.
(WebCore::CSSCursorImageValue::~CSSCursorImageValue): Call detachPendingImage.
(WebCore::CSSCursorImageValue::cachedImage): Call detachPendingImage before changing
m_image to a new value.
(WebCore::CSSCursorImageValue::clearCachedImage): Ditto.
* css/CSSCursorImageValue.h: Added detachPendingImage.

* css/CSSImageSetValue.cpp:
(WebCore::CSSImageSetValue::detachPendingImage): Added. Calls detachFromCSSValue
on the current image set if it is a StylePendingImage.
(WebCore::CSSImageSetValue::~CSSImageSetValue): Call detachPendingImage.
(WebCore::CSSImageSetValue::cachedImageSet): Call detachPendingImage before changing
m_imageSet to a new value.
* css/CSSImageSetValue.h: Added detachPendingImage.

* css/CSSImageValue.cpp:
(WebCore::CSSImageValue::detachPendingImage): Added. Calls detachFromCSSValue on the
current image if it is a StylePendingImage.
(WebCore::CSSImageValue::~CSSImageValue): Call detachPendingImage.
(WebCore::CSSImageValue::cachedImage): Call detachPendingImage before changing m_image
to a new value.
* css/CSSImageValue.h: Added detachPendingImage.

* rendering/style/StylePendingImage.h:
(WebCore::StylePendingImage::cssImageValue): Added a null check.
(WebCore::StylePendingImage::cssImageGeneratorValue): Added a null check.
(WebCore::StylePendingImage::cssCursorImageValue): Added a null check.
(WebCore::StylePendingImage::cssImageSetValue): Added a null check.
(WebCore::StylePendingImage::detachFromCSSValue): Added. Sets m_value to null since
the style is no longer using this StylePendingImage.
(WebCore::StylePendingImage::data): Changed to use the "this" pointer since all we
need is some arbitrary pointer uniquely identifying the image. Before loading the image,
we have no suitable weak identifier, so it suffices to use the unique pointer to each
StylePendingImage object. This function is used only in a limited way; it would be nice
to find a way to make the code less strange long term.

Change-Id: Iab8a5d033c5cdf9a488ac18d8f233dee48c5f3e7
git-svn-id: http://svn.webkit.org/repository/webkit/trunk@160479 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Reviewed-by: Michael Bruning <michael.bruning@digia.com>