Geolocation: log the number of wifi access points used to determine location.

content/browser/geolocation/network_location_request.cc

Index: content/browser/geolocation/network_location_request.cc
diff --git a/content/browser/geolocation/network_location_request.cc b/content/browser/geolocation/network_location_request.cc
index 9418a3f537d0917640606af8dec7989b25ff1592..082991a1ef769a577690a8ccda8bbdf687519245 100644
--- a/content/browser/geolocation/network_location_request.cc
+++ b/content/browser/geolocation/network_location_request.cc
@@ -58,6 +58,11 @@ void RecordUmaResponseCode(int code) {
       code);
 }
 
+void RecordUmaAccessPoints(int count) {
+  UMA_HISTOGRAM_SPARSE_SLOWLY("Geolocation.NetworkLocationRequest.AccessPoints",
+      count);

[Ilya Sherman, 2013/10/08 23:51:03]

What sorts of counts are expected?  Could you use a non-sparse histogram
instead?  A sparse histogram is likely to be less efficient for this use-case
than something like UMA_HISTOGRAM_COUNTS.

[Michael van Ouwerkerk, 2013/10/09 13:09:57]

I expect counts of 0 to 10 but in some cases there will be more. I want to use
buckets of size 1 for the range of 0 - 10.

I shied away from the macros in histogram.h because the comments state 0 is not
a possible value, it will be silently clamped to 1. Is this actually true?

[Ilya Sherman, 2013/10/09 22:47:40]

I'm pretty sure this is not accurate -- 0 is a valid bucket.  It's also the
implicit underflow bucket, but that shouldn't be an issue for this histogram. 
You can double-check this locally by emitting a 0 to the histogram, and then
navigate to chrome:histograms to see what values the histogram has.

[Michael van Ouwerkerk, 2013/10/10 10:20:03]

Done.

+}
+
 // Local functions
 // Creates the request url to send to the server.
 GURL FormRequestURL(const GURL& url);
@@ -107,6 +112,7 @@ bool NetworkLocationRequest::MakeRequest(const string16& access_token,
                                          const WifiData& wifi_data,
                                          const base::Time& timestamp) {
   RecordUmaEvent(NETWORK_LOCATION_REQUEST_EVENT_REQUEST_START);
+  RecordUmaAccessPoints(wifi_data.access_point_data.size());
   if (url_fetcher_ != NULL) {
     DVLOG(1) << "NetworkLocationRequest : Cancelling pending request";
     RecordUmaEvent(NETWORK_LOCATION_REQUEST_EVENT_REQUEST_CANCEL);