Add a missing BLINK_EXPORT macro to WebFrame::detach.

BUG=none
TBR=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/607473003

git-svn-id: svn://svn.chromium.org/blink/trunk@182709 bbb929c8-8fbe-4397-9dbb-9b2b20218538