Add a histogram to track HTTP auth cache size

This change adds a histogram that tracks the number of cache entries
examined when no matching entry is found (which is most of the time).
This will allow us to have a better proxy for HTTP auth cache size.

Bug: 757116
Change-Id: I8a4b7348a473d2802ac12dda2589569e523b3e5e
Reviewed-on: https://chromium-review.googlesource.com/c/1265435Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Lily Chen <chlily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597314}

net/http/http_auth_cache.cc

@@ -68,6 +68,11 @@ void RecordLookupByPathPosition(int position) {
   UMA_HISTOGRAM_COUNTS_100("Net.HttpAuthCacheLookupByPathPosition", position);
 }
 
+void RecordEntriesExaminedWhenNoMatch(int num_examined_entries) {
+  UMA_HISTOGRAM_COUNTS_100("Net.HttpAuthCacheEntriesExaminedWhenNoMatch",
+                           num_examined_entries);
+}
+
 }  // namespace
 
 namespace net {
@@ -94,6 +99,7 @@ HttpAuthCache::Entry* HttpAuthCache::Lookup(const GURL& origin,
     }
   }
   RecordLookupPosition(0);
+  RecordEntriesExaminedWhenNoMatch(entries_examined);
   return NULL;  // No realm entry found.
 }
 
@@ -128,6 +134,8 @@ HttpAuthCache::Entry* HttpAuthCache::LookupByPath(const GURL& origin,
   }
   if (best_match)
     best_match->last_use_time_ticks_ = tick_clock_->NowTicks();
+  else
+    RecordEntriesExaminedWhenNoMatch(entries_examined);
   RecordLookupByPathPosition(best_match_position);
   return best_match;
 }

tools/metrics/histograms/histograms.xml

@@ -54324,6 +54324,17 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Net.HttpAuthCacheEntriesExaminedWhenNoMatch">
+  <owner>chlily@chromium.org</owner>
+  <summary>
+    The number of entries examined (equal to the cache size) when attempting to
+    look up an HTTP auth cache entry (either by path or realm) and finding no
+    matching entries. This is recorded whenever the lookup fails (i.e., value 0
+    in Net.HttpAuthCacheLookupPosition and
+    Net.HttpAuthCacheLookupByPathPosition).
+  </summary>
+</histogram>
+
 <histogram name="Net.HttpAuthCacheLookupByPathPosition">
   <owner>asanka@chromium.org</owner>
   <summary>