Throttle network requests from installing service workers by default.

The trial showed that this is neutral in top level metrics like FCP.
This is somewhat expected as the throttling is intended as an additional
protection against a site contending with itself.

The trial also showed that install takes longer.  At the median installs
are ~10ms slower and at P99 are 2 to 5 seconds slower.  The P99 for
install is still less than 30 seconds, however, which places it well
under the 5 minute timeout threshold.  UMA also showed that while
timeouts increased slightly they still only occurred around ~0.1%
of the time.

Bug: 1035448
Change-Id: I40dbe69e363997725b29a282041e9e27aee9c712
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2376368
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801966}

third_party/blink/common/features.cc

@@ -650,11 +650,13 @@ const base::FeatureParam<AlignFontDisplayAutoTimeoutWithLCPGoalMode>
         &align_font_display_auto_timeout_with_lcp_goal_modes};
 
 // Enable throttling of fetch() requests from service workers in the
-// installing state.
+// installing state.  The limit of 3 was chosen to match the limit
+// in background main frames.  In addition, trials showed that this
+// did not cause excessive install delays or timeouts.
 const base::Feature kThrottleInstallingServiceWorker{
-    "ThrottleInstallingServiceWorker", base::FEATURE_DISABLED_BY_DEFAULT};
+    "ThrottleInstallingServiceWorker", base::FEATURE_ENABLED_BY_DEFAULT};
 const base::FeatureParam<int> kInstallingServiceWorkerOutstandingThrottledLimit{
-    &kThrottleInstallingServiceWorker, "limit", 5};
+    &kThrottleInstallingServiceWorker, "limit", 3};
 
 const base::Feature kInputPredictorTypeChoice{
     "InputPredictorTypeChoice", base::FEATURE_DISABLED_BY_DEFAULT};