Teach the network simulator to wait for Promises.

Before, it would continue as soon as the user's function returned, which might
not wait adequately for Promises in the user's code.

NOTRY=true

Review URL: https://codereview.chromium.org/536163002

git-svn-id: svn://svn.chromium.org/blink/trunk@181404 bbb929c8-8fbe-4397-9dbb-9b2b20218538