Revert 187427 "PPAPI: Remove threading options; it's always on"

ppapi/shared_impl/var_tracker.h

Index: ppapi/shared_impl/var_tracker.h
===================================================================
--- ppapi/shared_impl/var_tracker.h	(revision 187715)
+++ ppapi/shared_impl/var_tracker.h	(working copy)
@@ -10,8 +10,7 @@
 #include "base/basictypes.h"
 #include "base/hash_tables.h"
 #include "base/memory/ref_counted.h"
-#include "base/memory/scoped_ptr.h"
-#include "base/threading/thread_checker.h"
+#include "base/threading/non_thread_safe.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/c/pp_module.h"
 #include "ppapi/c/pp_var.h"
@@ -34,14 +33,16 @@
 // This class maintains the "track_with_no_reference_count" but doesn't do
 // anything with it other than call virtual functions. The interesting parts
 // are added by the PluginObjectVar derived from this class.
-class PPAPI_SHARED_EXPORT VarTracker {
+class PPAPI_SHARED_EXPORT VarTracker
+#ifdef ENABLE_PEPPER_THREADING
+    : NON_EXPORTED_BASE(public base::NonThreadSafeDoNothing) {
+#else
+    // TODO(dmichael): Remove the thread checking when calls are allowed off the
+    // main thread (crbug.com/92909).
+    : NON_EXPORTED_BASE(public base::NonThreadSafe) {
+#endif
  public:
-  // A SINGLE_THREADED VarTracker will use a thread-checker to make sure it's
-  // always invoked on the same thread on which it was constructed. A
-  // THREAD_SAFE VarTracker will check that the ProxyLock is held. See
-  // CheckThreadingPreconditions() for more details.
-  enum ThreadMode { SINGLE_THREADED, THREAD_SAFE };
-  explicit VarTracker(ThreadMode thread_mode);
+  VarTracker();
   virtual ~VarTracker();
 
   // Called by the Var object to add a new var to the tracker.
@@ -124,10 +125,6 @@
     ADD_VAR_CREATE_WITH_NO_REFERENCE
   };
 
-  // On the host-side, make sure we are called on the right thread. On the
-  // plugin side, make sure we have the proxy lock.
-  void CheckThreadingPreconditions() const;
-
   // Implementation of AddVar that allows the caller to specify whether the
   // initial refcount of the added object will be 0 or 1.
   //
@@ -175,12 +172,6 @@
   // a real WebKit ArrayBuffer on the host side.
   virtual ArrayBufferVar* CreateArrayBuffer(uint32 size_in_bytes) = 0;
 
-  // On the host side, we want to check that we are only called on the main
-  // thread. This is to protect us from accidentally using the tracker from
-  // other threads (especially the IO thread). On the plugin side, the tracker
-  // is protected by the proxy lock and is thread-safe, so this will be NULL.
-  scoped_ptr<base::ThreadChecker> thread_checker_;
-
   DISALLOW_COPY_AND_ASSIGN(VarTracker);
 };