gfx::Image: Added thread checker.

chrome/browser/themes/browser_theme_pack.h

Index: chrome/browser/themes/browser_theme_pack.h
diff --git a/chrome/browser/themes/browser_theme_pack.h b/chrome/browser/themes/browser_theme_pack.h
index adb608c78340019ca8dbf247c82a55a971cf17fb..3aa4f92663aab374d7a82646e1c62aaeaf2c39ab 100644
--- a/chrome/browser/themes/browser_theme_pack.h
+++ b/chrome/browser/themes/browser_theme_pack.h
@@ -261,9 +261,13 @@ class BrowserThemePack : public CustomThemeSupplier {
   ImageCache images_;
 
   // Cache of images created in BuildFromExtension(). Once the theme pack is
-  // created, this cache should only be accessed on the file thread. There
-  // should be no IDs in |image_memory_| that are in |images_on_file_thread_|
-  // or vice versa.
+  // created, this cache should only be accessed on the FILE thread (or whatever
+  // thread WriteToDisk is called from*). There should be no IDs in
+  // |image_memory_| that are in |images_on_file_thread_| or vice versa.
+  //
+  // *NOTE: These are usually accessed on the FILE thread, but as the
+  // documentation for WriteToDisk notes, it is not always the FILE thread; just
+  // some thread other than UI.
   ImageCache images_on_file_thread_;
 
   // Whether the theme pack has been succesfully initialized and is ready to