Implement StorageAreaProxy::getItem and StorageAreaProxy::setItem
https://bugs.webkit.org/show_bug.cgi?id=110927

Reviewed by Sam Weinig.

Source/WebCore:

Export StorageMap::getItem and StorageMap::setItem.

* WebCore.exp.in:

Source/WebKit2:

* UIProcess/Storage/StorageManager.cpp:
(WebKit::StorageManager::setItem):
* UIProcess/Storage/StorageManager.h:
Add empty stub.

* UIProcess/Storage/StorageManager.messages.in:
Add SetItem message.

* WebProcess/Storage/StorageAreaProxy.cpp:
(WebKit::StorageAreaProxy::getItem):
Fill the storage map and get the item from it.

(WebKit::StorageAreaProxy::setItem):
Update the storage map and send a SetItem message to the UI process.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144130 268f45cc-cd09-0410-ab3c-d52691b4dbfc