Revert of Oilpan: Prepare to move XPath::NodeSet, XPath::ValueData, and XPath::Value. (https://codereview.chromium.org/275053005/)

Reason for revert:
It might have caused a performance regression crbug.com/373814.

BUG=373814


Original issue's description:
> Oilpan: Prepare to move XPath::NodeSet, XPath::ValueData, and XPath::Value.
> 
> Note:
> - XPath::Value is a part object of XPathResult.
> - XPath::NodeSet is on-heap, a part object of XPath::ValueData and
> XPathResult, is copyable.
> 
> This is a preparation to remove RefPtr<Attr> and RefPtr<Node>.
> 
> BUG=357163
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=174061

TBR=oilpan-reviews@chromium.org,haraken@chromium.org,ager@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=357163

Review URL: https://codereview.chromium.org/284283003

git-svn-id: svn://svn.chromium.org/blink/trunk@174119 bbb929c8-8fbe-4397-9dbb-9b2b20218538