ASSERTION FAILED: m_purgePreventCount in FontCache::getCachedFontData running svg...

ASSERTION FAILED: m_purgePreventCount in FontCache::getCachedFontData running svg/custom/animate-disallowed-use-element.svg
https://bugs.webkit.org/show_bug.cgi?id=81002
<rdar://problem/11168969>

Reviewed by Simon Fraser.

Don't spend time rebuilding SVG text layout attributes if the document is being torn down, as
they won't be used, and can cause assertion failures in the font cache.

No new tests, as it fixes an intermittent assertion failure during document destruction.

* rendering/svg/RenderSVGInlineText.cpp:
(WebCore::RenderSVGInlineText::willBeDestroyed):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@112942 268f45cc-cd09-0410-ab3c-d52691b4dbfc