Bug 1773319 - Rework FrontendContex::gcSafeCallback so rooting hazard analysis understands it r=arai

Previously, rooting hazard analysis could report calls to gcSafeCallback as
triggering a GC because of the two calls to callback(). Now it understands that
the single call to callback() will not trigger a GC.

Differential Revision: https://phabricator.services.mozilla.com/D172333

js/src/frontend/FrontendContext.cpp

@@ -89,9 +89,9 @@ void FrontendContext::recoverFromOutOfMemory() {
 
 const JSErrorFormatString* FrontendContext::gcSafeCallback(
     JSErrorCallback callback, void* userRef, const unsigned errorNumber) {
+  mozilla::Maybe<gc::AutoSuppressGC> suppressGC;
   if (maybeCx_) {
-    gc::AutoSuppressGC suppressGC(maybeCx_);
-    return callback(userRef, errorNumber);
+    suppressGC.emplace(maybeCx_);
   }
 
   return callback(userRef, errorNumber);