Atomicize frequently identical ResourceResponse string members.

Atomicize frequently identical ResourceResponse string members.
<https://webkit.org/b/128887>

Store the mime type, text encoding and HTTP response status text in
AtomicStrings instead of Strings to deduplicate frequently occurring
values (e.g "text/html", "utf-8" and "OK".)

Reviewed by Geoffrey Garen.

* platform/network/ResourceResponseBase.h:
* platform/network/cf/ResourceResponseCFNet.cpp:
(WebCore::ResourceResponse::cfURLResponse):
(WebCore::ResourceResponse::platformLazyInit):
* platform/network/mac/ResourceResponseMac.mm:
(WebCore::ResourceResponse::platformLazyInit):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@164198 268f45cc-cd09-0410-ab3c-d52691b4dbfc