Check whether the marking deque overflowed in ProcessEphemeralMarking

Otherwise, we might exit to early when we call ProcessEphemeralMarking
with an empty, overflown deque

BUG=none
R=hpayer@chromium.org,erik.corry@gmail.com
LOG=n

Review URL: https://codereview.chromium.org/777643002

Cr-Commit-Position: refs/heads/master@{#25660}

src/heap/mark-compact.cc

@@ -2241,6 +2241,7 @@ void MarkCompactCollector::MarkLiveObjects() {
         &IsUnmarkedHeapObject);
     // Then we mark the objects.
     heap()->isolate()->global_handles()->IterateWeakRoots(&root_visitor);
+    ProcessMarkingDeque();
 
     // Repeat Harmony weak maps marking to mark unmarked objects reachable from
     // the weak roots we just marked as pending destruction.