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

components/error_page/common/net_error_info.h

Index: components/error_page/common/net_error_info.h
diff --git a/components/error_page/common/net_error_info.h b/components/error_page/common/net_error_info.h
index 071bbd640107d683c76c1314f5aca20e104d8c88..bc1b28821065aa948b824977371cfd143de153d7 100644
--- a/components/error_page/common/net_error_info.h
+++ b/components/error_page/common/net_error_info.h
@@ -33,6 +33,12 @@ enum NetworkErrorPageEvent {
 
   NETWORK_ERROR_EASTER_EGG_ACTIVATED,            // Easter egg activated.
 
+  // For "Google cached copy" button.
+  NETWORK_ERROR_PAGE_CACHED_COPY_BUTTON_SHOWN,
+  NETWORK_ERROR_PAGE_CACHED_COPY_BUTTON_CLICKED,
+  NETWORK_ERROR_PAGE_CACHED_PAGE_BUTTON_SHOWN,
+  NETWORK_ERROR_PAGE_CACHED_PAGE_BUTTON_CLICKED,

[mmenke, 2015/07/20 18:14:27]

Is the only difference here the label?  I don't think that's at all clear from
the code here, in net_error_helper_core.h, or in localized_error.cc.

Suggest some comment making it clearer.  Maybe 'For "Google cached copy"
button.' -> 'For "Google cached copy" button label experiment.'  Suggest doing
something similar at the other locations.

[mmenke, 2015/07/20 18:14:27]

These are presumably going to be removed at some point.  To avoid making the
histogram a weird mix of old and new values, that gives us two options:

1)  Add numbers to the events, and when events are removed, comment out the old
values, so new histograms won't re-use the values.
2)  Rename the histogram whenever we change it.  This has the upside of not
mixing data from the two different histograms.  Not as important here, as this
is more of an event count than an actual histogram, but does make sure we're
comparing comparable values.

[edwardjung, 2015/07/21 11:43:37]

Makes sense. Added comments.

[edwardjung, 2015/07/21 11:43:37]

Yes these would be removed once the experiment would be over. I'll implement
option 1 for now. 

+
   NETWORK_ERROR_PAGE_EVENT_MAX,
 };