Export WebCore::forgetV8ObjectForNPObject

This is required for de-associating an NPObject from an existing V8 wrapper,
so we don't pull any of the wrapper's custom properties into the context of
a new page when navigating to it.

BUG=372914

Review URL: https://codereview.chromium.org/289473008

git-svn-id: svn://svn.chromium.org/blink/trunk@174083 bbb929c8-8fbe-4397-9dbb-9b2b20218538