Bug 1407143 (Part 4) - Reset lastMarkSlice earlier r=jonco

This change isn't really required but may tighten some other conditions /
assertions.

--HG--
extra : rebase_source : 84a96811e0f01df80f897137768f10c292442584

js/src/gc/GC.cpp

@@ -7095,7 +7095,7 @@ GCRuntime::incrementalCollectSlice(SliceBudget& budget, JS::gcreason::Reason rea
         initialReason = reason;
         cleanUpEverything = ShouldCleanUpEverything(reason, invocationKind);
         isCompacting = shouldCompact();
-        lastMarkSlice = false;
+        MOZ_ASSERT(!lastMarkSlice);
         rootsRemoved = false;
 
         incrementalState = State::MarkRoots;
@@ -7156,7 +7156,7 @@ GCRuntime::incrementalCollectSlice(SliceBudget& budget, JS::gcreason::Reason rea
         }
 
         incrementalState = State::Sweep;
-
+        lastMarkSlice = false;
         beginSweepPhase(reason, session);
 
         MOZ_FALLTHROUGH;