Blink: Adds a mechanism to pause the serviceworker startup sequence after downloading the main script and then to resume. This is to give the browser process a chance to compare the old and new script during a softupdate. 

BUG=371671

Review URL: https://codereview.chromium.org/339703006

git-svn-id: svn://svn.chromium.org/blink/trunk@176552 bbb929c8-8fbe-4397-9dbb-9b2b20218538