Fix when fast/events/message-port-gc-closed.html forces a GC.

Delay the garbage collection until the event handler invocation has returned.

R=haraken
BUG=420061

Review URL: https://codereview.chromium.org/635473002

git-svn-id: svn://svn.chromium.org/blink/trunk@183263 bbb929c8-8fbe-4397-9dbb-9b2b20218538