Extensions: Add UMA to monitor whether a webview request has DWR rules to be evaluated.

extensions/browser/api/web_request/web_request_api.cc

Index: extensions/browser/api/web_request/web_request_api.cc
diff --git a/extensions/browser/api/web_request/web_request_api.cc b/extensions/browser/api/web_request/web_request_api.cc
index ea2a7fea7713de6878366df9cf7b61412c991007..334f3f409a0e63a08fa7d65f39fcc265459e73a1 100644
--- a/extensions/browser/api/web_request/web_request_api.cc
+++ b/extensions/browser/api/web_request/web_request_api.cc
@@ -1999,6 +1999,13 @@ bool ExtensionWebRequestEventRouter::ProcessDeclarativeRules(
     return true;
   }
 
+  if (is_web_view_guest) {
+    const bool has_declarative_rules = !relevant_registries.empty();
+    UMA_HISTOGRAM_BOOLEAN(
+        "Extensions.DeclarativeWebRequest.WebViewRequestDeclarativeRules",

[lazyboy, 2017/05/05 17:46:45]

relevant_registries.empty() == true doesn't necessarily mean that webview has
applicable rules, it only means it might have applicable rules in the registry,
right? Because if deltas_created is false below, then it wouldn't have any rules
to apply.
I'd change the description/name in relevant places to reflect that.

Also I'd be clear about the motivation of this UMA in CL description.

[karandeepb, 2017/05/05 18:40:27]

Have removed the usage of the word applicable and changed it to clarify that the
request has rules that need to be evaluated. Have also updated the CL
description.

+        has_declarative_rules);
+  }
+
   base::Time start = base::Time::Now();
 
   bool deltas_created = false;