Oilpan: clear FrameLoader during LocalFrame detach.

Following r183834, the FrameOwner is disconnected during frame detach.
The clearing of the FrameLoader that needs to happen for Oilpan can
consequently be moved to a more natural place and be done as part
of the other LocalFrame detach operations (rather than being a
special action of FrameOwner disconnecting.)

R=haraken
BUG=425559

Review URL: https://codereview.chromium.org/678673002

git-svn-id: svn://svn.chromium.org/blink/trunk@184382 bbb929c8-8fbe-4397-9dbb-9b2b20218538