Reviewed by Darin Adler.

        Should make use of purge priorities for different resource types
        https://bugs.webkit.org/show_bug.cgi?id=48684

        Assign purge priorities to CachedResource objects. JavaScript and CSS stylesheets are more
        important than images so hint to the kernel to purge them last. We do this by repurposing
        the previously unused function, PurgeableBuffer::setPurgePriority(). It now just sets the
        priority field and does not have side-effects like calling makePurgeable(true).

        Also, remove unusued PurgeableBuffer::create(const Vector<char>&) function and move the
        purgeable memory size threshold from CachedResource to PurgeableBuffer.

        * WebCore.xcodeproj/project.pbxproj: Add new PurgePriority.h header and mark it as private
        header.
        * loader/CachedCSSStyleSheet.h:
        (WebCore::CachedCSSStyleSheet::purgePriority):
        * loader/CachedImage.h:
      ...