Bug 1422264 - Don't switch to non-incremental GC mode if it's already non-incremental r=sfink

--HG--
extra : rebase_source : 81e04ff2dcec60ec4bf0625faaaf219cdf24a159
extra : amend_source : 372f79945ff90c6bca96e604771d000cb8332b12

js/src/jsgc.cpp

@@ -6953,7 +6953,7 @@ GCRuntime::incrementalCollectSlice(SliceBudget& budget, JS::gcreason::Reason rea
             AutoGCRooter::traceAllWrappers(target, &marker);
 
         /* If we needed delayed marking for gray roots, then collect until done. */
-        if (!hasBufferedGrayRoots()) {
+        if (isIncremental && !hasBufferedGrayRoots()) {
             budget.makeUnlimited();
             isIncremental = false;
             stats().nonincremental(AbortReason::GrayRootBufferingFailed);