Fix inspector crash when reloading during a debugger pause on a page with unload event handlers

This was regressed in r155659.

BUG=279919
TEST=manually tested with repro in bug

Review URL: https://chromiumcodereview.appspot.com/23640006

git-svn-id: svn://svn.chromium.org/blink/trunk@157375 bbb929c8-8fbe-4397-9dbb-9b2b20218538