Add UMA for disk latency for cache accesses.

BUG=487749

Review URL: https://codereview.chromium.org/1134353003

Cr-Commit-Position: refs/heads/master@{#329857}

net/disk_cache/simple/simple_synchronous_entry.cc

@@ -14,9 +14,11 @@
 #include "base/files/file_util.h"
 #include "base/hash.h"
 #include "base/location.h"
+#include "base/metrics/histogram.h"
 #include "base/numerics/safe_conversions.h"
 #include "base/sha1.h"
 #include "base/strings/stringprintf.h"
+#include "base/timer/elapsed_timer.h"
 #include "net/base/io_buffer.h"
 #include "net/base/net_errors.h"
 #include "net/disk_cache/simple/simple_backend_version.h"
@@ -216,6 +218,7 @@ void SimpleSynchronousEntry::OpenEntry(
     const uint64 entry_hash,
     bool had_index,
     SimpleEntryCreationResults *out_results) {
+  base::ElapsedTimer open_time;
   SimpleSynchronousEntry* sync_entry =
       new SimpleSynchronousEntry(cache_type, path, "", entry_hash);
   out_results->result =
@@ -230,6 +233,7 @@ void SimpleSynchronousEntry::OpenEntry(
     out_results->stream_0_data = NULL;
     return;
   }
+  UMA_HISTOGRAM_TIMES("SimpleCache.DiskOpenLatency", open_time.Elapsed());
   out_results->sync_entry = sync_entry;
 }
 

tools/metrics/histograms/histograms.xml

@@ -37425,6 +37425,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="SimpleCache.DiskOpenLatency" units="milliseconds">
+  <owner>rdsmith@chromium.org</owner>
+  <summary>
+    The time to access (open and read headers from all files) the cache entry on
+    disk.
+  </summary>
+</histogram>
+
 <histogram name="SimpleCache.EntryCreationResult" enum="BooleanSuccess">
   <obsolete>
     Deprecated 2013-08 in favor of cache type specific version (App or Http).