Ensure parent is accessible in WebFrameClient::frameDetached() callback.

Some of the out-of-process iframe CLs in Chrome depend on being able to
query parent() from the callback, but that didn't work after r164390 for
two reasons:
1) Frame is cleared before calling into WebFrameClient, so it's
   impossible for WebFrame to traverse to Frame's parent.
2) The FrameLoaderClient itself was being invoked after the frame had
   already been unlinked from the frame tree.

BUG=331426
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/123213005

git-svn-id: svn://svn.chromium.org/blink/trunk@164529 bbb929c8-8fbe-4397-9dbb-9b2b20218538