Simple code cleanup in WindowProxy.
Simple code cleanup in WindowProxy.

- updateDocument() unconditionally dereferences LocalFrame::document(),
  so there's no point in trying to protect it against being null further
  down (both if checks will always pass in the context it's being called
  from)
- Remove the redundant call to updateSecurityToken(), since
  updateDocument() does the same thing.
- Use Frame::securityContext() instead of Frame::document() to get to
  the security origin, since WindowProxy will wrap a Frame* in the
  future.

BUG=425623

Review URL: https://codereview.chromium.org/769123003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@186273 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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