Use CheckUserHooks::getClientIPfromXFF when reading XFF header

When we read the XFF header from the current WebRequest, use
the same function the CheckUser uses internally when validating
XFF headers.

Change-Id: I23dcdef6163c75ab44fc663efa0a8e062b0a6c57
Merge "Use CheckUserHooks::getClientIPfromXFF when reading XFF header"