Map: Guard concurrent accesses to a std::map by a lock in drawing code.

chrome/browser/renderer_host/accelerated_surface_container_manager_mac.h

Index: chrome/browser/renderer_host/accelerated_surface_container_manager_mac.h
diff --git a/chrome/browser/renderer_host/accelerated_surface_container_manager_mac.h b/chrome/browser/renderer_host/accelerated_surface_container_manager_mac.h
index bbfa490e73b8d84b9977909e16be12919f6523e9..0e38c1194f18041675055764d0c51ca2ecb46df4 100644
--- a/chrome/browser/renderer_host/accelerated_surface_container_manager_mac.h
+++ b/chrome/browser/renderer_host/accelerated_surface_container_manager_mac.h
@@ -11,6 +11,7 @@
 
 #include "app/surface/transport_dib.h"
 #include "base/basictypes.h"
+#include "base/lock.h"
 #include "gfx/native_widget_types.h"
 
 namespace webkit_glue {
@@ -106,6 +107,11 @@ class AcceleratedSurfaceContainerManagerMac {
   // created, this is set to |false| while the compositor is not needed.
   bool gpu_rendering_active_;
 
+  // Both |plugin_window_to_container_map_| and the
+  // AcceleratedSurfaceContainerMac in it are not threadsafe, but accessed from
+  // multiple threads. All these accesses are guarded by this lock.
+  mutable Lock lock_;
+
   DISALLOW_COPY_AND_ASSIGN(AcceleratedSurfaceContainerManagerMac);
 };