Copy stack until safepoint *before* notifying other threads at safepoints.

R=vegorov@chromium.org
BUG=356118

Review URL: https://codereview.chromium.org/218683007

git-svn-id: svn://svn.chromium.org/blink/trunk@170528 bbb929c8-8fbe-4397-9dbb-9b2b20218538