Bug 1883457 - Part 1: Disable jit compilation within the parent process on iOS, r=jandem

This adds a pref to disable jit compilation by calling JS::DisableJitBackend in
the parent process. This will be used on iOS, where the jit entitlements are
only available for sandboxed content processes.

Differential Revision: https://phabricator.services.mozilla.com/D203498

modules/libpref/init/StaticPrefList.yaml

@@ -7960,6 +7960,12 @@
   value: false
   mirror: always
 
+# Whether to disable the jit within the main process
+- name: javascript.options.main_process_disable_jit
+  type: bool
+  value: @IS_IOS@
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "layers."
 #---------------------------------------------------------------------------

xpcom/build/XPCOMInit.cpp

@@ -233,6 +233,11 @@ static void InitializeJS() {
   JS::SetAVXEnabled(mozilla::StaticPrefs::javascript_options_wasm_simd_avx());
 #endif
 
+  if (XRE_IsParentProcess() &&
+      mozilla::StaticPrefs::javascript_options_main_process_disable_jit()) {
+    JS::DisableJitBackend();
+  }
+
   // Set all JS::Prefs.
   SET_JS_PREFS_FROM_BROWSER_PREFS;