Revert 186925 since it breaks many of the builds on the bots (e.g.

webkit/plugins/ppapi/host_var_tracker.cc

Index: webkit/plugins/ppapi/host_var_tracker.cc
===================================================================
--- webkit/plugins/ppapi/host_var_tracker.cc	(revision 186938)
+++ webkit/plugins/ppapi/host_var_tracker.cc	(working copy)
@@ -16,8 +16,7 @@
 namespace webkit {
 namespace ppapi {
 
-HostVarTracker::HostVarTracker()
-  : VarTracker(SINGLE_THREADED) {
+HostVarTracker::HostVarTracker() {
 }
 
 HostVarTracker::~HostVarTracker() {
@@ -28,7 +27,7 @@
 }
 
 void HostVarTracker::AddNPObjectVar(NPObjectVar* object_var) {
-  CheckThreadingPreconditions();
+  DCHECK(CalledOnValidThread());
 
   InstanceMap::iterator found_instance = instance_map_.find(
       object_var->pp_instance());
@@ -48,7 +47,7 @@
 }
 
 void HostVarTracker::RemoveNPObjectVar(NPObjectVar* object_var) {
-  CheckThreadingPreconditions();
+  DCHECK(CalledOnValidThread());
 
   InstanceMap::iterator found_instance = instance_map_.find(
       object_var->pp_instance());
@@ -73,7 +72,7 @@
 
 NPObjectVar* HostVarTracker::NPObjectVarForNPObject(PP_Instance instance,
                                                     NPObject* np_object) {
-  CheckThreadingPreconditions();
+  DCHECK(CalledOnValidThread());
 
   InstanceMap::iterator found_instance = instance_map_.find(instance);
   if (found_instance == instance_map_.end())
@@ -88,7 +87,7 @@
 }
 
 int HostVarTracker::GetLiveNPObjectVarsForInstance(PP_Instance instance) const {
-  CheckThreadingPreconditions();
+  DCHECK(CalledOnValidThread());
 
   InstanceMap::const_iterator found = instance_map_.find(instance);
   if (found == instance_map_.end())
@@ -97,7 +96,7 @@
 }
 
 void HostVarTracker::DidDeleteInstance(PP_Instance instance) {
-  CheckThreadingPreconditions();
+  DCHECK(CalledOnValidThread());
 
   InstanceMap::iterator found_instance = instance_map_.find(instance);
   if (found_instance == instance_map_.end())