[flags] Add a flag to independently enable/disable Atomics

- Adds a --harmony-atomics feature to gate Atomics. This allows us to
  independently add SharedArrayBuffer and Atomics on the global object
  of Contexts, which is necessary for migration to a COOP+COEP site
  isolation requirement.

Bug: chromium:923807
Change-Id: If80c12eb86dc0251a5e5fad62a6dd5ced3380b5b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2340322
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70129}

src/flags/flag-definitions.h

@@ -276,6 +276,7 @@ DEFINE_IMPLICATION(harmony_weak_refs_with_cleanup_some, harmony_weak_refs)
   V(harmony_namespace_exports,                                            \
     "harmony namespace exports (export * as foo from 'bar')")             \
   V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")               \
+  V(harmony_atomics, "harmony atomics")                                   \
   V(harmony_import_meta, "harmony import.meta property")                  \
   V(harmony_dynamic_import, "harmony dynamic import")                     \
   V(harmony_promise_all_settled, "harmony Promise.allSettled")            \

src/init/bootstrapper.cc

@@ -4151,6 +4151,12 @@ void Genesis::InitializeGlobal_harmony_sharedarraybuffer() {
 
   JSObject::AddProperty(isolate_, global, "SharedArrayBuffer",
                         isolate()->shared_array_buffer_fun(), DONT_ENUM);
+}
+
+void Genesis::InitializeGlobal_harmony_atomics() {
+  if (!FLAG_harmony_atomics) return;
+
+  Handle<JSGlobalObject> global(native_context()->global_object(), isolate());
 
   JSObject::AddProperty(isolate_, global, "Atomics",
                         isolate()->atomics_object(), DONT_ENUM);