Bug 1623565 - P4: Make browser.cache.check_doc_frequency a StaticPref. r=nika

Due to the refactored code,
browser/base/content/test/performance/browser_preferences_usage.js started
reporting more accesses to browser.cache.check_doc_frequency preference.

Differential Revision: https://phabricator.services.mozilla.com/D68319

docshell/base/nsDocShellLoadState.cpp

@@ -813,7 +813,7 @@ nsLoadFlags nsDocShellLoadState::CalculateChannelLoadFlags(
     case LOAD_NORMAL:
     case LOAD_LINK:
       // Set cache checking flags
-      switch (Preferences::GetInt("browser.cache.check_doc_frequency", -1)) {
+      switch (StaticPrefs::browser_cache_check_doc_frequency()) {
         case 0:
           loadFlags |= nsIRequest::VALIDATE_ONCE_PER_SESSION;
           break;

modules/libpref/init/StaticPrefList.yaml

@@ -787,6 +787,16 @@
   value: 50
   mirror: always
 
+# How often to validate document in cache
+#   0 = once-per-session,
+#   1 = each-time,
+#   2 = never,
+#   3 = when-appropriate/automatically
+- name: browser.cache.check_doc_frequency
+  type: RelaxedAtomicUint32
+  value: 3
+  mirror: always
+
 - name: browser.contentblocking.database.enabled
   type: bool
   value: false

modules/libpref/init/all.js

@@ -233,8 +233,6 @@ pref("browser.bookmarks.max_backups",       5);
 pref("browser.cache.disk.capacity",         256000);
 
 pref("browser.cache.disk_cache_ssl",        true);
-// 0 = once-per-session, 1 = each-time, 2 = never, 3 = when-appropriate/automatically
-pref("browser.cache.check_doc_frequency",   3);
 // The half life used to re-compute cache entries frecency in hours.
 pref("browser.cache.frecency_half_life_hours", 6);