Simplify jettisoning of style resolvers on memory pressure.

Simplify jettisoning of style resolvers on memory pressure.
<https://webkit.org/b/129644>

Now that we have Document::allDocuments(), we can jettison all the
style resolvers by walking the set of live documents and calling
clearStyleResolver() on each one, instead of having a function on
Page that walks the frame tree of every living page, etc.

Reviewed by Antti Koivisto.

* page/Page.cpp:
* page/Page.h:
* platform/mac/MemoryPressureHandlerMac.mm:
(WebCore::MemoryPressureHandler::releaseMemory):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165629 268f45cc-cd09-0410-ab3c-d52691b4dbfc