service worker: Add UseCounter for importScripts() after main script installation

Currently we allow importScripts() to load a new script after service
worker's installation. According to the spec we should throw a TypeError
in such case. We want to change the behavior but it seems that some
sites depend on the current behavior so changing the behavior is a bit
risky at this point. This CL add a UseCounter to track such usage.

Bug: 852666, 719052
Change-Id: I3e5748a7ce037be75f4f2dfa965d8cb34df9b9bc
Reviewed-on: https://chromium-review.googlesource.com/1126799
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573209}
3 files changed