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

ash/screenshot_delegate.h

Index: ash/screenshot_delegate.h
diff --git a/ash/screenshot_delegate.h b/ash/screenshot_delegate.h
index 51a74dbd5e70a04b2e96da2158397eaf7cd8fef9..fe194e8a792d599b0435ea62d3dc862362f5a89f 100644
--- a/ash/screenshot_delegate.h
+++ b/ash/screenshot_delegate.h
@@ -29,6 +29,9 @@ class ScreenshotDelegate {
   // window.
   virtual void HandleTakePartialScreenshot(
       aura::Window* window, const gfx::Rect& rect) = 0;
+
+  // Returns true if the system is ready to take screenshot.
+  virtual bool CanTakeScreenshot() = 0;
 };
 }  // namespace ash