Shrink FontDescription object size

Shrink FontDescription object size
https://bugs.webkit.org/show_bug.cgi?id=86950

Reviewed by Alexey Proskuryakov.

Reduce object size of FontDescription by 8 bytes on 64-bit, 4 bytes on 32-bit systems.
Note that this means Font, StyleInheritedData and RenderSVGInlineText shrink too.

* platform/graphics/FontDescription.cpp:
* platform/graphics/FontDescription.h:
(WebCore::FontDescription::script):
(FontDescription):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@117868 268f45cc-cd09-0410-ab3c-d52691b4dbfc