Oilpan: remove RawPtr<> usage for WebNotification.

Go the whole hog and use (non-const) bare pointers for the conversion
constructor and assignment operator.

R=
BUG=340522

Review URL: https://codereview.chromium.org/329463004

git-svn-id: svn://svn.chromium.org/blink/trunk@175875 bbb929c8-8fbe-4397-9dbb-9b2b20218538