Revert 167612 "Remove unused WebPrivatePtr::operator=(std::nullp..."

Results in undefined symbol on Linux.
http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20%28dbg%29/builds/17859

> Remove unused WebPrivatePtr::operator=(std::nullptr_t).
> 
> With std::nullptr_t constructors in place for the various Ptr types,
> this method is no longer needed.
> 
> Also revert some reset() uses that were introduced while waiting for
> the same nullptr change to land.
> 
> R=abarth@chromium.org
> BUG=
> 
> Review URL: https://codereview.chromium.org/173713008

TBR=sigbjornf@opera.com

Review URL: https://codereview.chromium.org/169763006

git-svn-id: svn://svn.chromium.org/blink/trunk@167615 bbb929c8-8fbe-4397-9dbb-9b2b20218538