sdch: add Sdch3.AdvertisedWithSecureScheme histogram

This histogram tracks how frequently SDCH is advertised over a secure scheme.

BUG=488086

Review URL: https://codereview.chromium.org/1137753006

Cr-Commit-Position: refs/heads/master@{#329921}

net/base/sdch_manager.cc

@@ -282,6 +282,8 @@ SdchManager::GetDictionarySet(const GURL& target_url) {
     return NULL;
 
   UMA_HISTOGRAM_COUNTS("Sdch3.Advertisement_Count", count);
+  UMA_HISTOGRAM_BOOLEAN("Sdch3.AdvertisedWithSecureScheme",
+                        target_url.SchemeIsSecure());
 
   return result.Pass();
 }

tools/metrics/histograms/histograms.xml

@@ -35298,6 +35298,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Sdch3.AdvertisedWithSecureScheme" enum="BooleanHttps">
+  <owner>ellyjones@chromium.org</owner>
+  <summary>
+    Whether an SDCH dictionary was advertised over a secure scheme or not. This
+    histogram is logged inside SdchManager at advertisement time.
+  </summary>
+</histogram>
+
 <histogram name="Sdch3.Advertisement_Count">
   <owner>rdsmith@chromium.org</owner>
   <summary>