Change behavior of skipWaiting()

Before this, skipWaiting() promises when called multiple times on a
waiting worker had different behaviors among each other where the winner
waits unitl the Activate is complete while others don't. This makes the
behavior consistent by making the Activate be called only once. To
achieve that, this removes the steps in skipWaiting() that call the
Activate on a waiting worker (so promise resolves right away after
setting the skip waiting flag) and simplifies the call sites of the
Activate in the Install algorithm.

Related issue: #1015.

docs/index.bs

@@ -814,8 +814,6 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
         1. Let |promise| be a new <a>promise</a>.
         1. Run the following substeps <a>in parallel</a>:
             1. Set [=/service worker=]'s <a>skip waiting flag</a>.
-            1. If [=/service worker=]'s <a>state</a> is *installed*, then:
-                1. Run <a>Activate</a> algorithm passing [=/service worker=]'s [=service worker/registration=] as the argument.  
             1. Resolve |promise| with undefined.
         1. Return |promise|.
     </section>
@@ -2578,13 +2576,10 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
       1. Run the <a>Update Registration State</a> algorithm passing |registration|, "<code>installing</code>" and null as the arguments.
       1. Run the <a>Update Worker State</a> algorithm passing |registration|'s <a>waiting worker</a> and *installed* as the arguments.
       1. If |redundantWorker| is not null, run the <a>Update Worker State</a> algorithm passing |redundantWorker| and *redundant* as the arguments.
-      1. If |registration|'s <a>waiting worker</a>'s <a>skip waiting flag</a> is set, then:
-          1. Run <a>Activate</a> algorithm passing |registration| as the argument.
-          1. Invoke <a>Finish Job</a> with |job| and abort these steps.
       1. Invoke <a>Finish Job</a> with |job|.
       1. Wait for all the <a>tasks</a> <a lt="queue a task">queued</a> by <a>Update Worker State</a> invoked in this algorithm have executed.
       1. Wait until no [=/service worker client=] is <a>using</a> |registration| or |registration|'s <a>waiting worker</a>'s <a>skip waiting flag</a> is set.
-      1. If |registration|'s <a>waiting worker</a> |waitingWorker| is not null and |waitingWorker|'s <a>skip waiting flag</a> is not set, invoke <a>Activate</a> algorithm with |registration| as its argument.
+      1. If |registration|'s <a>waiting worker</a> is not null, invoke <a>Activate</a> algorithm with |registration| as its argument.
   </section>
 
   <section algorithm>