Bug 1693223 - Improve gray marking assertions to take account of clearing mark bits off-thread r=sfink

I don't know whether this will fix this issue, but our assertions are
definitely over restrictive here.

The mark bits are accessed atomically, and are cleared on a background thread.
If the mark bits for a cell span two words (which happens for some cell sizes)
we may observe a cell that had both mark bits set (i.e. black) as having only
one bit set, and thus appear to be gray.

The patch checks whether parallel unmarking is taking place and skips the gray
check.

Differential Revision: https://phabricator.services.mozilla.com/D154794

js/src/shell/js.cpp

@@ -8052,6 +8052,8 @@ static bool GetMarks(JSContext* cx, unsigned argc, Value* vp) {
     JSObject* obj = observers->get()[i];
     if (!obj) {
       color = "dead";
+    } else if (obj->zone()->isGCPreparing()) {
+      color = "unmarked";
     } else {
       gc::TenuredCell* cell = &obj->asTenured();
       if (cell->isMarkedGray()) {