Bug 1622111 - Convert dom.datatransfer.mozAtAPIs to static pref. r=njn

Converts dom.datatransfer.mozAtAPIs to a static pref and adds the `do_not_use_directly` field, since the pref is used in combination with an additional check in `DataTransfer::MozAtAPIsEnabled`.

Differential Revision: https://phabricator.services.mozilla.com/D67202

--HG--
extra : moz-landing-system : lando

dom/events/DataTransfer.cpp

@@ -44,8 +44,6 @@
 #include "nsNetUtil.h"
 #include "nsReadableUtils.h"
 
-#define MOZ_CALLS_ENABLED_PREF "dom.datatransfer.mozAtAPIs"
-
 namespace mozilla {
 namespace dom {
 
@@ -1545,17 +1543,9 @@ void DataTransfer::SetMode(DataTransfer::Mode aMode) {
 
 /* static */
 bool DataTransfer::MozAtAPIsEnabled(JSContext* aCx, JSObject* aObj /*unused*/) {
-  // Read the pref
-  static bool sPrefCached = false;
-  static bool sPrefCacheValue = false;
-
-  if (!sPrefCached) {
-    sPrefCached = true;
-    Preferences::AddBoolVarCache(&sPrefCacheValue, MOZ_CALLS_ENABLED_PREF);
-  }
-
   // We can expose moz* APIs if we are chrome code or if pref is enabled
-  return nsContentUtils::IsSystemCaller(aCx) || sPrefCacheValue;
+  return nsContentUtils::IsSystemCaller(aCx) ||
+         StaticPrefs::dom_datatransfer_mozAtAPIs_DoNotUseDirectly();
 }
 
 }  // namespace dom

modules/libpref/init/StaticPrefList.yaml

@@ -1460,6 +1460,14 @@
   value: false
   mirror: always
 
+# Disable moz* APIs in DataTransfer.
+# Should be accessed via DataTransfer::MozAtAPIsEnabled.
+- name: dom.datatransfer.mozAtAPIs
+  type: bool
+  value: false
+  mirror: always
+  do_not_use_directly: true
+
 # Any how many seconds we allow external protocol URLs in iframe when not in
 # single events
 - name: dom.delay.block_external_protocol_in_iframes

modules/libpref/init/all.js

@@ -4708,9 +4708,6 @@ pref("dom.noopener.newprocess.enabled", true);
 // loops with faulty converters involved.
 pref("general.document_open_conversion_depth_limit", 20);
 
-// Disable moz* APIs in DataTransfer
-pref("dom.datatransfer.mozAtAPIs", false);
-
 pref("fission.rebuild_frameloaders_on_remoteness_change", true);
 
 // Support for legacy customizations that rely on checking the