[Sync] Make syncer commands avoid posting tasks on threads with no work to do

chrome/browser/sync/engine/model_changing_syncer_command.h

Index: chrome/browser/sync/engine/model_changing_syncer_command.h
diff --git a/chrome/browser/sync/engine/model_changing_syncer_command.h b/chrome/browser/sync/engine/model_changing_syncer_command.h
index 9a2311e96d907365d7f61a6cd519aa431f962cc3..2c747530936160157efe18d566377ae5fd6b9cf5 100644
--- a/chrome/browser/sync/engine/model_changing_syncer_command.h
+++ b/chrome/browser/sync/engine/model_changing_syncer_command.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include "base/compiler_specific.h"
+#include "chrome/browser/sync/engine/model_safe_worker.h"
 #include "chrome/browser/sync/engine/syncer_command.h"
 #include "chrome/browser/sync/util/unrecoverable_error_info.h"
 
@@ -41,6 +42,31 @@ class ModelChangingSyncerCommand : public SyncerCommand {
     return UnrecoverableErrorInfo();
   }
 
+  std::set<ModelSafeGroup> GetGroupsToChangeForTest(
+      const sessions::SyncSession& session) const {
+    return GetGroupsToChange(session);
+  }
+
+ protected:
+  // Hack to track down which subclass triggers the perf regression.
+  // (See comments in http://codereview.chromium.org/8637006/ for
+  // details.)  If this returns false (the default),
+  // GetGroupsToChange() is not used and session.GetEnabledGroups()
+  // is used instead.
+  //
+  // TODO(akalin): Remove this when we track down the perf regression.
+  virtual bool HasCustomGroupsToChange() const = 0;
+
+  // This should return the set of groups in |session| that need to be
+  // changed.  The returned set should be a subset of
+  // session.GetEnabledGroups().  Subclasses can guarantee this either
+  // by calling one of the session.GetEnabledGroups*() functions and
+  // filtering that, or using GetGroupForModelType() (which handles
+  // top-level/unspecified nodes) to project from model types to
+  // groups.
+  virtual std::set<ModelSafeGroup> GetGroupsToChange(
+      const sessions::SyncSession& session) const = 0;
+
   // Sometimes, a command has work to do that needs to touch global state
   // belonging to multiple ModelSafeGroups, but in a way that is known to be
   // safe.  This will be called once, prior to ModelChangingExecuteImpl,