Move dispatchBeforeUnloadEvent to WebFrame, part 1/3.

BUG=357782
TEST=no change

Review URL: https://codereview.chromium.org/217543005

git-svn-id: svn://svn.chromium.org/blink/trunk@170407 bbb929c8-8fbe-4397-9dbb-9b2b20218538