Bug 1470280 - Part 1: Make browser_preferences_usage.js less brittle. r=kmag

browser_preferences_usage.js hardcodes a max value of 15 accesses which doesn't scale well as we increase the number of processes. Instead we'll base the max on a multiplier of the default process count which should avoid bustage in the future but still catch egregious accesses.

--HG--
extra : rebase_source : 5484be4833dfc220126f893c722c793e7be93ed1
extra : source : 60699c2387f6e51398c345996a97551a867bd2d7

browser/base/content/test/performance/browser_preferences_usage.js

@@ -1,6 +1,8 @@
 /* Any copyright is dedicated to the Public Domain.
    http://creativecommons.org/publicdomain/zero/1.0/ */
 
+const DEFAULT_PROCESS_COUNT = Services.prefs.getDefaultBranch(null).getIntPref("dom.ipc.processCount");
+
 /**
  * A test that checks whether any preference getter from the given list
  * of stats was called more often than the max parameter.
@@ -120,7 +122,10 @@ add_task(async function startup() {
 
 // This opens 10 tabs and checks pref getters.
 add_task(async function open_10_tabs() {
-  let max = 15;
+  // This is somewhat arbitrary. When we had a default of 4 content processes
+  // the value was 15. We need to scale it as we increase the number of
+  // content processes so we approximate with 4 * process_count.
+  const max = 4 * DEFAULT_PROCESS_COUNT;
 
   let whitelist = {
     "layout.css.dpi": {
@@ -146,10 +151,6 @@ add_task(async function open_10_tabs() {
       min: 10,
       max: 18,
     },
-    "dom.ipc.processCount": {
-      min: 10,
-      max: 15,
-    },
     "browser.startup.record": {
       max: 20,
     },