bug 1442776 make CycleCollectedJSContext accessible from JSContext private r=peterv

Inheriting PerThreadAtomCache on CycleCollectedJSContext permits use of
static_cast, avoiding one level of indirection compared to adding a
CycleCollectedJSContext* to PerThreadAtomCache.

PerThreadAtomCache is over 18kB, and so WorkerJSContext and WorkletJSContext
are moved from the stack to the heap.

MozReview-Commit-ID: 6jdJeZcviK4

dom/plugins/base/nsNPAPIPlugin.cpp

@@ -20,6 +20,7 @@
 #include "nsPluginStreamListenerPeer.h"
 #include "nsIServiceManager.h"
 #include "nsThreadUtils.h"
+#include "mozilla/CycleCollectedJSContext.h" // for nsAutoMicroTask
 #include "mozilla/Preferences.h"
 #include "nsPluginInstanceOwner.h"