Create StorageClient, move more DOMStorage implementation into core

This eliminates a layering violation wherein the implementation of
StorageNamespace lived in Source/web.

Storage{Area,Namespace}Proxy are renamed without their "Proxy" suffix,
but otherwise the code remains basically the same.

A further refactor could probably be done to eliminate the WebCore
classes altogether, and just use WebStorage{Area,Namespace} directly
from core, but I leave that for another patch (in particular because
such a change would make the callsites in core somewhat clumsier).

R=abarth@chromium.org, michaeln@chromium.org

Review URL: https://codereview.chromium.org/159263002

git-svn-id: svn://svn.chromium.org/blink/trunk@166871 bbb929c8-8fbe-4397-9dbb-9b2b20218538