Refactor GpuDataManagerImpl to make it thread-safe.

content/public/browser/gpu_data_manager_observer.h

Index: content/public/browser/gpu_data_manager_observer.h
===================================================================
--- content/public/browser/gpu_data_manager_observer.h	(revision 200103)
+++ content/public/browser/gpu_data_manager_observer.h	(working copy)
@@ -35,6 +35,9 @@
   // Called for any observer when the GPU process crashed.
   virtual void OnGpuProcessCrashed(base::TerminationStatus exit_code) {}
 
+  // Called for any observer when the system switches to a different GPU.
+  virtual void OnGpuSwitching() {}
+
  protected:
   virtual ~GpuDataManagerObserver() {}
 };