Add UMA histograms for Google cached copy experiment on network error interstitial

chrome/renderer/net/net_error_page_controller.h

Index: chrome/renderer/net/net_error_page_controller.h
diff --git a/chrome/renderer/net/net_error_page_controller.h b/chrome/renderer/net/net_error_page_controller.h
index 20be3ee0b04693c55adc5496d4269d1d8717fd19..dc50a6666c60343493ecee50fa9291cc8ce8f424 100644
--- a/chrome/renderer/net/net_error_page_controller.h
+++ b/chrome/renderer/net/net_error_page_controller.h
@@ -43,6 +43,9 @@ class NetErrorPageController
   // Track easter egg plays.
   bool TrackEasterEgg();
 
+  // Track "Show cached copy/page" button clicks.
+  bool TrackCachedCopyButtonClick(bool is_default_label);
+
   // Track a click when the page has suggestions from the navigation correction
   // service.
   bool TrackClick(const gin::Arguments& args);