Bug 1582348 - Enable writable streams in the browser when the javascript.options.{,writable_}streams prefs are set. (Writable streams are only half-implemented; DO NOT start reporting bugs yet, it *will* crash in all sorts of trivial ways.) r=jandem

Differential Revision: https://phabricator.services.mozilla.com/D51049

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

js/xpconnect/src/XPCJSContext.cpp

@@ -772,6 +772,8 @@ void xpc::SetPrefableRealmOptions(JS::RealmOptions& options) {
   options.creationOptions()
       .setSharedMemoryAndAtomicsEnabled(sSharedMemoryEnabled)
       .setStreamsEnabled(sStreamsEnabled)
+      .setWritableStreamsEnabled(
+          StaticPrefs::javascript_options_writable_streams())
       .setFieldsEnabled(sFieldsEnabled)
       .setAwaitFixEnabled(sAwaitFixEnabled);
 }

modules/libpref/init/StaticPrefList.yaml

@@ -4016,6 +4016,16 @@
   value: true
   mirror: always
 
+# Writable Streams API.  (The pref above must also be set to expose this.)
+#
+# Writable streams are still EXTRAORDINARILY BETA and it is well-known that
+# things are likely pretty broken if you poke much at all, so if you flip this
+# preference, don't report bugs against it just yet.
+- name: javascript.options.writable_streams
+  type: RelaxedAtomicBool
+  value: false
+  mirror: always
+
 - name: javascript.options.main_thread_stack_quota_cap
   type: uint32_t
 #if defined(MOZ_ASAN)