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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176552

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

git-svn-id: svn://svn.chromium.org/blink/trunk@176666 bbb929c8-8fbe-4397-9dbb-9b2b20218538