Ignores screenshot taking invocations if it's too close to the previous one.

ash/accelerators/accelerator_filter_unittest.cc

Index: ash/accelerators/accelerator_filter_unittest.cc
diff --git a/ash/accelerators/accelerator_filter_unittest.cc b/ash/accelerators/accelerator_filter_unittest.cc
index 723960a20020fda901947b3db8157229f4a58234..af376b770945ed1ef741e5130719bb231c4c0045 100644
--- a/ash/accelerators/accelerator_filter_unittest.cc
+++ b/ash/accelerators/accelerator_filter_unittest.cc
@@ -40,6 +40,10 @@ class DummyScreenshotDelegate : public ScreenshotDelegate {
     // Do nothing because it's not tested yet.
   }
 
+  virtual bool CanTakeScreenshot() OVERRIDE {
+    return true;
+  }
+
   int handle_take_screenshot_count() const {
     return handle_take_screenshot_count_;
   }