Adds kind-of-live thumbnail generation for a potential tab switcher.

chrome/browser/renderer_host/backing_store_manager.h

Index: chrome/browser/renderer_host/backing_store_manager.h
diff --git a/chrome/browser/renderer_host/backing_store_manager.h b/chrome/browser/renderer_host/backing_store_manager.h
index 5b929f6e7141a7be98e0efd1c32f4ca6f621f794..3078a9358e92bfa268ab460a9db54f1b917d6c49 100644
--- a/chrome/browser/renderer_host/backing_store_manager.h
+++ b/chrome/browser/renderer_host/backing_store_manager.h
@@ -58,6 +58,11 @@ class BackingStoreManager {
   // Removes the backing store for the host.
   static void RemoveBackingStore(RenderWidgetHost* host);
 
+  // Expires the given backing store. This emulates something getting evicted
+  // from the cache for the purpose of testing. Returns true if the host was
+  // removed, false if it wasn't found.
+  static bool ExpireBackingStoreForTest(RenderWidgetHost* host);
+
  private:
   // Not intended for instantiation.
   BackingStoreManager() {}