Report the wasted network bytes when resource prefetches hit max limit

BUG=625270

Review-Url: https://codereview.chromium.org/2278793002
Cr-Commit-Position: refs/heads/master@{#418646}

components/precache/core/precache_fetcher.cc

@@ -280,6 +280,10 @@ void PrecacheFetcher::Fetcher::OnURLFetchDownloadProgress(
     response_bytes_ = network_response_bytes_ = current;
     was_cached_ = source->WasCached();
 
+    UMA_HISTOGRAM_CUSTOM_COUNTS("Precache.Fetch.ResponseBytes.NetworkWasted",
+                                network_response_bytes_, 1,
+                                1024 * 1024 /* 1 MB */, 100);
+
     // Cancel the download.
     network_url_fetcher_.reset();
     callback_.Run(*this);

tools/metrics/histograms/histograms.xml

@@ -45291,6 +45291,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Precache.Fetch.ResponseBytes.NetworkWasted" units="bytes">
+  <owner>rajendrant@chromium.org</owner>
+  <owner>bengr@chromium.org</owner>
+  <summary>
+    The total number of response bytes received over the network for a wasted
+    resource precache fetch. Logged when a partially downloaded resource
+    precache fetch gets cancelled due to per-resource size limit or max precache
+    size limit.
+  </summary>
+</histogram>
+
 <histogram name="Precache.Fetch.ResponseBytes.Total" units="bytes">
   <owner>twifkak@chromium.org</owner>
   <owner>bengr@chromium.org</owner>